STFC
MPI für Kohlenforschung

University College London

The DL_POLY Interface

Introduction

The following chapter describes the molecular mechanics interface, which allows the setting up of MM force-field expressions of the CHARMM, AMBER, GROMOS, UFF and (approximate) MM2 types. As for the other energy and gradient routines, this is accomplished by the implementation of a family of routines, including dl_poly.init, dl_poly.energy, and dl_poly.gradient (see description of the general energy/gradient interface for more details). Most of the arguments described below must be passed to the initialisation function dl_poly.init and will be ignored if passed the energy and gradient procedures.

   
Relationship to the DL_POLY Project

The implementation of the MM functionality makes use of some code from the DL_POLY package that is currently under development by Dr W. Smith at CCLRC Daresbury Laboratory, and the support of the authors of DL_POLY is gratefully acknowledged. Users should note however that the interface described in this chapter is not the one that will be used by the DL_POLY project and any bugs present in the current implementation are most likely to be result of errors in the interface and parameterisation, rather than DL_POLY itself, and as such are not the responsibility of the DL_POLY project. Not all of the features in the release of DL_POLY_2 are available through the ChemShell interface, so this chapter should not be regarded as a description of the functionality of the DL_POLY package. For detailled infromation, there is an online version of the manual of DL_POLY_2 (pdf, 1.6 MB).

Options to dl_poly.init

The table indicates the control arguments that may be passed to dl_poly.init.

Argument Argument type Mandatory Default To specify
coords= Fragment tag yes none Initial configuration of the system
conn= Fragment tag no use coords= Connectivity to use
mm_defs= file yes none specify file for force-field definition
use_pairlist= Boolean no yes Use pairlist (alternative is all-pairs n**2 loop)
no_elec= Boolean no no Electrostatic potential terms off
cutoff= real no 99.99 Ang Cutoff for pairlist generation (in a.u.). In case of periodic systmes, the default is half the cell size.
mxlist= integer no 5000 Allocation parameter for pairlist, allow max(natoms,mxlist) entries per atom
mxexcl= integer no variable Allocation parameter for excluded atom list, may need to be increased for qm/mm calculations with a large qm region
exact_srf= Boolean no no Evaluate short-range forces exactly, rather than using lookup tables. Not available for periodic systems
scale14= Tcl list of 2 integers no {1.0 1.0} Scale factors for electrostatic and VDW interactions, respectively
atom_types= Tcl List no natoms*undef Atom type settings (undef means auto generate)
atom_charges= Tcl List no natoms*unset Atom charges (unset means take forcefield value for the type)
debug_rank_e= Boolean no off request ranked energy contributions debug
list_option= Output keyword no full Output verbosity
debug_pairlist= Boolean no off request tabulation of pair and excluded atoms list
debug_times= Boolean no off print timing information
debug_memory= Boolean no off request listing of size of DL_POLY arrays
ecut= real number no not applied maximum acceptable value for the energy
frozen= Tcl List no none Specify a list of frozen atoms. Energy and force contributions between such atoms are not calculated. This changes the total energy by a constant amount.
groups= Tcl List no none Specify a list of (ideally neutral) charge groups. A list of lists, each sublist containing the atom numbers of atom belonging to the group. Increases the accuracy of the electrostatic energy when using a cutoff, see electrostatics keywords.
spme= Boolean no yes Use Smoothed Particle Mesh Ewald Summation to calculate the electrostatics. If "no", standard ewald summation is used. This keyword is ignored if no ewald sum is done, see electrostatics keywords.
ewald_precision= integer no 6 Set the estimated relative accuracy of the Ewald (or SPME) summation to 10-N where N is spcified by ewald_precision.
suppress_ewald= Boolean no no Set to yes for periodic systmes if you want to suppress to calculate the electrostatics via Ewald summation. The default for periodic systems is Ewald summation (SPME).
forcefield= keyword no none Use forcefield=uff to apply the UFF force field.

The mm_defs argument specifies a file containing a keyword-based definition of the force-field, and atom type information. For details of the keywords that may be used see below.

The (optional) conn= argument is not required if the connect function, invoked with the default tolerance settings, produces a satisfactory connectivity for use in the parameterisation.

The (optional) constrain= argument at present may take only a single keyword qc, and is used in the pre-optimisation of the MM part of a QC calculation. It specifies that a set of harmonic distance constraints be applied between pairs of atoms lying within the qc part of the molecule. The use of this option required that the qc part of the molecule be defined by applying the appropriate attributes (qc and junction) to the atom types that represent QC atoms. The harmonic force constant (atomic units) may be specified using the constrain= argument.

Specifying the electrostatics

The treatment of the electrostatic interaction is generally the most demanding part of a molecular dynamics simulation. Thus it requires special attention. The following options are chosen depending on the provided keywords:
  1. Direct coulomb sum is used when no neighbour list is calculated (use_pairlist=no). Accurate but demanding.
  2. Atomistic cutoff is used whenever a cutoff is specified and no Ewald sum is done (see below). This option is NOT recommended, as it leads to spurious energy changes when the neighbour list is updated (call to dl_poly.update).
  3. Charge group implementation is used when neutral groups are specified (groups=), a cutoff is used, and no Ewald sum is done. Pair interactions are excluded on a neutral group basis. This method is recommended whenever SPME is not possible.
  4. Ewald sum requires periodic systems. Used for all periodic systems if spme=no and suppress_ewald=no (default).
  5. Smoothed Particle Mesh Ewald summation (SPME) requires periodic systems. The most accurate and efficient method to calculate electrostatic interactions. Recommended whenever possible, default for periodic systems.
For further information, the reader is referred to the manual of DL_POLY (pdf, 1.6MB).

CHARMM simulation mode

DL_POLY can take the topology and the force field parameters from a PSF file generated from CHARMM and the corresponding parameter and topology files.

Argument Argument type Mandatory Default To specify
use_charmm_psf= Boolean no no Whether to use topology (PSF) from CHARMM
charmm_psf_file= file yes (See note 1) - Topology (PSF) from CHARMM
charmm_parameter_file= file yes (See note 1) - CHARMM parameters (.inp format)
charmm_mass_file= file yes (See note 1) - CHARMM atomic masses (RTF format)
Notes

1. These arguments are mandatory only if using a CHARMM PSF.

Using the CHARMM PSF will usually also require setting the atom_types= and atom_charges= arguments.

It is sometimes helpful to read the connectivity from the CHARMM PSF file into a fragment file (here charmm.c):

load_connect_from_psf charmm.c myfile.psf

If VMD is used to create the PSF file rather than the CHARMM program, the PSF file results in XPLOR format. It can be read in by
read_xplor_psf atom_types= types atom_charges= charges atom_masses= mass file= myfile.psf
which reads the atom types into a variable called types, the atom charges into a variable called charges, and the masses into a variable called mass. These can subsequently used in arguments to dl_poly as: 

   scale14 = { 1.0 1.0 } \
   atom_types= $types \
   atom_charges= $charges \
   use_charmm_psf=yes \
   charmm_psf_file= myfile.psf \
   charmm_parameter_file= par_all27_prot_lipid.prm \
   charmm_mass_file= top_all27_prot_lipid.rtf ]

AMBER simulation mode

DL_POLY can take the topology and the force field parameters from prmtop and inpcrd files generated from AMBER.

Argument Argument type Mandatory Default To specify
amber_prmtop_file= file no no Whether to use topology (prmtop) from AMBER, specifies the filename of the prmtop file

To read the amber coordinates into a ChemShell fragment, the following command can be used:

load_amber_coords inpcrd=<inpcrd file> prmtop=<prmtop file> coords=<fragment name>

   
Setting up the Force Field

The following entries are made in the file specified by the mm_defs option.

   
Defining the Atom Types

By default, elemental atom types are defined automatically, as is a wild type (*) that may be used for general parameter assignment.

It is possible to define new atom types, and this is necessary for the parameterisation of systems in which a number of inequivalent environments exist for the same elemental type.

To facilitate the definition of the force field in terms of generic and specific parameter types, a hierarchical definition of atom types is used.

In general, definition of new types may be performed in response to a number of keywords.

declare
It is simply declared, with optional specification of a legal connectivity count for items of this type. It is simply declared, with optional specification of a legal connectivity count for items of this type.
declare <type> <# conn> <# conn> ..
subgroup
It is simply declared as a subtype of an existing atom type, using the subgroup keyword.
subgroup <existing-type> <type> <description>
e.g. subgroup c ct ``Amber ca carbon type''
declares ct to be a new type, a subtype of c. The descriptive string is used by the log file to identify the type.
supergroup
It is derived by combining a number of existing types using the supergroup command. Specifies that the new atom type is to be considered as a subtype of an existing type.
supergroup <type>
substructure query
It is defined using a substructure query. The substructure query is specified by a group of keyword-type fields as follows.

group

Define a new atom type as a superset of existing groups. 

group  <type> <description> <type1> <type2> ..

query

Commence definition of an atom type. 
query <type> <description>

target

Define an atom type specification that an atom must satisfy if it is to be classified as a member of the type we are defining with this query. This command effectively defines the first atom of the substructure query. 
target <type>

atom

This keyword adds another atom to the substructure query, to match the atom must have the type specified. 
atom <type>

connect

Specify that a pair of atoms (denoted by their order of definition using target and atom) must be connected for a substructure match. 
connect <index1> <index2>

endquery

Specifying the Atom Types

The atom types will be set up by default to the atomic types, or according to any substructure-based rules you read in. It is usually simpler for a small molecule, to define the atom types by hand, rather than to specify rules for automatic typing. Atom types are read in using the keyword types, followed by a list of types, one for each atom. The special type ``free'' may be specified, in which case the automatic types will be used. Note that unless rules have been provided to distinguish between atom types that refer to the same element, automatic typing will fail. A repeat count (of the form count*type ) may be used for brevity when the same type applies to a number of consecutive atoms.

   
Specifying the Energy Terms

The following entries may be included in a file (specified by the mm_defs argument) and serve to define a force field. In all cases, whether a given term is used in the parameterisation of the molecule in question is a function of the atom types assigned to the atoms of the molecule, and those present in the force field definition.

The general rule is that if a group of atoms in the molecule have the correct types and the correct pattern of connectivity to match the force-field definition, then that force-field term will be applied to those atoms.

Bonds

 

bond
 

$V = k(r-r_0)^2$

Format:  bond <type1><type2><k><r0> The force constant <k> is specified in kcal mol-1 Å-2, and the equilibrium distance <r0> in Å. It will be applied only to connected pairs of atoms with atom types that are the same as (or subgroups of) the types<type1> and <type2>.

mm2bond
 

$V = \frac{k}{2} (r - r_{0})^{2} (1 - 2(r - r_{0}))$

Format:  bond <type1><type2><k><r0>

The force constant <k> is specified in mdyn Å-1, and the equilibrium distance <r0> in Å. It will be applied only to connected pairs of atoms with atom types that are the same as (or subgroups of) the types<type1> and <type2>.

NOTE: Note that in this implementation no special precautions are taken to deal with the steep fall in energy at long distances due to the cubic term. Poor structures should be pre-optimised using a harmonic bond stretch forcefield first.

quarbond
 

$V = k2 (r - r_{0})^{2} + k3 (r - r_{0})^{3} + k4 (r - r_{0})^{4}$

Format:  bond <type1><type2><k2><k3><k4><r0> The force constants <kn> are specified in kcal mol-1 Å-n, and the equilibrium distance <r0> in Å. It will be applied only to connected pairs of atoms with atom types that are the same as (or subgroups of) the types<type1> and <type2>.

Angles

 

angle
 

$V = k (\theta - \theta_{0})^{2}$

Format:  angle <type1><type2><type3><k> $\theta_{0}$

The force constant <k> is specified in kcal mol-1 rad-2, and the equilibrium angle $\theta_{0}$ in degrees. It will be applied only to connected sets of three atoms (connected 1-2-3) with atom types that are the same as (or subgroups of) the types <type1>, <type2> and <type3>.

ubangle
  

$V = k_{bend} (\theta - \theta_{0})^{2} + k_{stretch} (r-r_0)^2$

Format:  ubangle <type1> <type2> <type3> <kbend> <$\theta_{0}$ > <kstretch> <r0>

ubangle is used to define a combined harmonic bond angle term and Urey-Bradley term (1-3 harmonic non-bonded interaction). The force constant <kbend> is specified in kcal mol-1 rad-2, and the equilibrium angle $\theta_{0}$ in degrees. The force constant <kstretch> is specified in kcal mol-1 Å-2, and the equilibrium distance <r0> in Å. It will be applied only to connected sets of three atoms (connected 1-2-3; note that <r> is the distance between the unconnected atoms 1 and 3) with atom types that are the same as (or subgroups of) the types <type1>, <type2> and <type3>.

mm2angle
  

$V = \frac{k}{2} (\theta - \theta_{0})^{2} + k_{sb} (\theta - \theta_{0}) ((r - r_{0}) + (r' - r'_{0}))$

where r and r' denote the lengths of the bonds participating in the angle.

Format:  mm2angle <type1><type2><type3><k> $\theta_{0}$

The MM2 angle bend expression is used, and a stretch-bend interaction is added if ksb is non-zero. It is an fatal error to input a mm2angle term for an angle a-b-c if stretching terms for the a-b and b-c bonds are not also defined. The force constant and stretch-bend constant are in the millidyne-based units used by MM2 (see JACS v99, (1977) p8130).

NOTE: The implementation is not exactly equivalent to that implemented in the MM2 program, as the stretch-bend coupling term is retained for angles involving H atoms (in MM2 such stretch-bend terms are deleted).

quarangle
 

$V = k2 (\theta - \theta_{0})^{2} + k3 (\theta - \theta_{0})^{3} + k4 (\theta - \theta_{0})^{4}$

Format:  bond <type1><type2><k2><k3><k4> $\theta_{0}$

The force constants <kn> are specified in kcal mol-1 rad-n, and the equilibrium angle $\theta_{0}$ in degrees. It will be applied only to connected sets of three atoms (connected 1-2-3) with atom types that are the same as (or subgroups of) the types <type1>, <type2> and <type3>.

Torsions

 

ptor
  

$V(n) = {kn} (1 + cos (n \phi - \delta_{n}) )$

where $\phi$ is the torsion angle.

Format:  ptor <type1><type2><type3><type4><kn> $\delta_{n}$ <n><key><flip flag>

ptor is used to define a periodic torsional potential term, consisting of a series of up to three cosine terms.

key determines the connectivity of the four atoms, and is used when choosing sets of four atoms for which the potential will be calculated, as shown in the table below.

<key> connection Comments
i-j-k-l i-j, j-k, k-l conventional proper torsion angle
k-ijl k-i, k-j, k-l improper definition (as used in AMBER)
i-jkl i-j, i-k, i-l improper definition (as used in GROMOS)

flip flag determines whether the torsional angle may be flipped, or is defined in a fixed sense. The default value is 0, meaning it may be flipped.

The angle is measured around the vector j-k.

If more that one record is provided with the same <type1> -<type4> and<key> the parameters are combined to give a composite torsional potential. No more than 3 terms may be specified for a composite potential. $\delta$ should be provided in degrees.

mm2ptor
 

$V = k1 (1 + cos \phi ) + k2 (1 - cos 2\phi ) + k3 (1 + cos 3\phi )$

where $\phi$ is the torsion angle.

Format:

 mm2ptor <type1><type2><type3><type4><k1><k2><k3>

mm2ptor is used to input a proper torsional potential of MM2 form.

The kn are given in kcal mol-1.

mm2tor
 

$V = \frac{k1}{2} (1 + cos \phi ) + \frac{k2}{2} (1 - cos 2\phi ) + \frac{k3}{2} (1 + cos 3\phi )$

$\phi$ is the torsion angle.

Format:

 mm2tor <type1><type2><type3><type4><k1><k2><k3>

mm2tor is used to input a proper torsional potential of MM2 form.

The kn are given in kcal mol-1.

htor
 

$V = k (\phi - \phi_{0})^2$

$\phi$ is the torsion angle.

 htor <type1><type2><type3><type4><k> $\phi_{0}$ <key>

htor is used to define a harmonic torsion term, and usually used to apply planarity or tetrahedral constraints.

The force constant <k> is specified in kcal mol-1 rad-2 and $\phi_{0}$ is in degrees. <key> specifies the connectivity of the four atoms as indicated above for the ptor definition.

When the harmonic torsion parameters are applied to a given set of four atoms, the value of $\phi_{0}$ provided will be multiplied by -1.0 if doing so reduces the computed energy of the initial configuration.

cfftor
 

$V = k1 (1 - cos \phi) + k2 (1 - cos 2\phi) + k3 (1 - cos 3\phi)$

$\phi$ is the torsion angle.

Format:  cfftor <type1><type2><type3><type4><k1><k2><k3><key>

cfftor is used to define the consistent force field torsional potential term.

key determines the connectivity of the four atoms, and is used when choosing sets of four atoms for which the potential will be calculated, as shown in the table above. The angle is measured around the vector j-k.

Bonded coupling potentials

 

bb-ba-couple
 

$V = k_{ba} (\theta - \theta_{0}) (r - r_{0}) + k_{b^{\prime}a} (\theta - \theta_{0}) (r^\prime - r^\prime_{0}) + k_{bb^\prime} (r - r_{0}) (r^\prime - r^\prime_{0})$

Format:  bb-ba-couple <type1><type2><type3><kba><kb'a><kbb'>

bb-ba-couple is used to input a generic bond-bond and bond-angle coupling. The units of the force constants are kcal mol-1Å-1 rad-1 for the bond-angle terms and kcal mol-1Å-2 for the bond-bond coupling term. Note that the bond-angle coupling terms may be discriminated between, contrary to the mm2angle term (see 1.4.3). Bond b is assumed to be between atoms of <type1> and <type2>; bond b' between atoms of <type2> and <type3>.

NOTE The program will create an error if a regular angle or one of the bonds participating in the angle is not specified prior to definition of the coupling term.

aa-couple
 

$V = k_{aa^\prime} (\theta - \theta_{0}) (\theta^\prime -\theta^\prime_{0}) + k_{aa^{\prime\prime}} (\theta - \theta_{0}) (\theta^{\prime\prime} -\theta^{\prime\prime}_{0}) + k_{a^\prime a^{\prime\prime}} (\theta^\prime -\theta^\prime_{0}) (\theta^{\prime\prime} -\theta^{\prime\prime}_{0})$

Format:  aa-couple <type1><type2><type3><type4><kaa'> <kaa''> <ka'a''>

aa-couple is used to input angle-angle coupling terms. The input is organised as follows: the atom common to all angles is first (<type1>), followed by the connected atoms. Angle a is between<type2>-<type1>-<type3>, angle a' is between<type2>-<type1>-<type4>, and angle a'' is between<type3>-<type1>-<type4>.

NOTE The program will create an error if the regular angles are not specified prior to definition of the coupling term.

aat-couple
 

$V = k cos \phi (\theta - \theta_{0}) (\theta^\prime -\theta^\prime_{0})$

Format:  aat-couple <type1><type2><type3><type4><k>

aat-couple is used to input angle-angle-torsion coupling terms, and the types are given as for proper torsion input (see 1.4.3). Angles $\theta$ and $\theta^\prime$ are between <type1>-<type2>-<type3> and <type2>-<type3>-<type4>, respectively.

NOTE The program will create an error if the regular proper torsion or either of the regular angles are not specified prior to definition of the coupling term.

Non-bonded potentials

 

vdw
 

$V = - \frac{c6}{r^{6}} + \frac{c12}{r^{12}}$

Format:  vdw <type1><type2><c6><c12>

The units of <c6> are kcal mol-1 Å-6, those for <c12> are kcal mol-1 Å-12.

powers
 

$V = \frac{ck}{r^{k}} + \frac{cl}{r^{l}} + \frac{cm}{r^{m}}$

Format:  powers <type1><type2><ck><cl><cm>

powers is used to input general 3-term power-law non-bonded potentials.

The units of <cn> are kcal mol-1 Å-n.

NOTE k,l, and m need not necessarily be integer numbers.

m_n_vdw
 

$V = \frac{\varepsilon}{m-n} (n (\frac{r_{0}}{r})^{m} - m (\frac{r_{0}}{r})^{n})$

Format:  m_n_vdw <type1><type2><m><n><r0> < $\varepsilon$ >

The units for <r0> are Å, and for <$\varepsilon$ > kcal mol-1.

exp_6_vdw
 

$V = a exp (-\frac{r}{r_{0}}) - \frac{c6}{r^{6}}$

Format:  exp_6_vdw <type1><type2><a><r0><c6>

exp_6_vdw is used for Buckingham (exp-6) type potentials.

<r0> is given in Å, <a> in kcal mol-1, and <c6> in kcal mol-1 Å-6.

mm2_vdw
  ?????

Format:  mm2_vdw <type1><type2><rvdw> <$\varepsilon$ ><r0>

mm2_vdw is a special keyword allowing the input of Buckingham (exp-6) type potentials using the MM2 parameterisation.

<rvdw> is the sum of van der Waals radii for the two centres in Å, <$\varepsilon$ > is an energy parameter (kcal/mol) and <r0> should be set to 0.0 until the shifted H-coordinates used in MM2 are implemented.

   
The Excluded Atom List

The excluded atom list in DL_POLY will include

  • Those generated implicitly within DL_POLY from the presence of bond and angle parameters. Note that it is the presence of an assigned parameter (rather than a geometrical bonded contact). Therefore, if you decide you don't want to include a MM bond term between two atoms, simply leaving the term out of the definition of the force-field will result a short-range force and electrostatic term being evaluated, which may not be what you want.
  • Attributes of specific atom types (see below). These generate entries in an internal table (printed in the listing), which is implemented by generating zero force-constant bonds. These serve to avoid the inclusion of non-bonded when parameters are systematically omitted, as in the case of hybrid calculations.

       
Specifying Attributes

The parameterisation may be modified by giving some of the atoms or atom types specific attributes, which modify the parameters assigned to atoms of the relevant type. The attributes may be assigned either to the atom types, or to the individual atoms.

Setting attributes by atom type: attribute

To set the attributes associated with a given atom type, use the keyword attribute. 

attribute <atom-type> <attribute>

   
Setting attributes by atom : atom_attributes

To set the attribute for specific atoms, use the atom_attributes keyword. This method of input is designed for cases where you wish to associate either no attributes, or a single one, with each atom. One attribute entry is given for each atom, the special attributes ``unset'' is used where no attribute is to be set. As for the types keyword described above, you may use repeat counts of the form count*attribute. 

atom_attributes <attribute list>

For example, to associate the qc attribute with the first 2 atoms of a 8 atom molecule, you could use: 

atom_attributes 2*qc 6*unset

     
qc and junction

The attribute qc may be applied to designate an atom within the QC part (QM part) of a hybrid QM/MM calculation (see the hybrid module).

The attribute junction may be applied to designate an atom which lies within the QC part of a hybrid MM/QC calculation, but has bonds to atoms described by molecular mechanics.

This will have the effect of partially suppressing the parameterisation of atoms of the types that have the attributes specified. No bond terms including two qc or junction atoms, or angle terms with a qc or junction atom in the central position will be included.

Proper torsion angles around bonds wholly within the QC component will be deleted, as will improper torsional terms with a connectivity pattern i-j-k-l. Improper torsion terms with connectivity i-j,k,l or k-i,j,l will be deleted if the cental atom (i or k respectively) is part of the QC component.

An entry in the excluded atom list will be generated between all atoms within the QC component, and between all pairs of atoms that are bonded to the same junction atom.

   
exclude

The attribute exclude may be used to remove non-bonded forces between a group of atoms by setting the exclude attribute for all atom types in the group. All atoms with atom types for which this attribute is set will have zero force-constant bonds introduced to all other such atoms. This will force MM to generate entries in the excluded atom list.

       
Specifying Charges

It is possible to change the charge used in the electrostatics calculation either by setting a a charge associated with an atom type, or setting the atomic charges individually.

charge

This keyword sets the atomic charge associated with a given atom type. 

charge <type> <charge>

atom_charges

The keyword atom_charges may be used to specify a list of charges, to be applied to each individual atom. One charge entry must be given per atom, the word ``unset'' should be used for an atom if the charge is to be determined from the atom type.

For example, the following record will set the MM charge of the first atom to -1.0, the remainder will be have charges determined by their atom type. 

atom_charges -1.0 3*unset

Please note that the inclusion of electrostatic terms is independent of the inclusion of the vdw (or similar) short-range forces. They will be included for all charged atoms unless an excluded-atom entry is preset (see above).

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