QM interfaces

This section describes the options in common to most QM interfaces.

In addition to these options, each QM interface may contain options specific to that code. For details please see the individual interface manual pages:

General options

basis

(default: '3-21g') Basis set name or a filename (e.g., 'my_basis.txt') of plain text containing basis set parameters. In the latter case, a basis set file can be generated from the Basis Set Exchange.

Py-ChemShell also supports the BSE (Basis Set Exchange) plugin. For installation please see Installing ChemShell. In this case, the basis keyword argument can take a ChemShell BSE instance as the value. For example, to simply specify a basis set name for all atoms:

my_basis = addons.BSE('3-21g')
my_qm = NWChem(frag=my_frag, method='dft', basis=my_basis)

Or to use a hybrid basis set for various elements by a dict (both element symbol and atomic number are accepted):

my_basis = addons.BSE({1:'3-21g', 'O':'6-31g*'})

Or a hybrid basis set for various atom names (especially useful for calculating the finite cluster model):

my_basis = addons.BSE({'Mg1':'cc-pvdz','O1':'aug-cc-pVDZ'})

with a dict specifying hybrid basis sets for various atom names. Or:

charge

(default: 0) Charge to be used in the QM calculation

Allowed types of value:

  • A Python int for an integer charge

  • A Python float what will be rounded to the nearest integer charge

  • A Python str being 'automatic' or 'dl_field' makes ChemShell determine the charge by summing up the formal charges (typically useful when charges are not initially available but assigned later by DL_FIELD in a QM/MM calculation)

ecp

(default: '') A filename (e.g., 'my_ecp.txt') of plain text containing effective core potential (ECP) parameters.

functional

(default: 'blyp') Choice of exchange-correlation functional in DFT calculations

method

Allowed values:

  • 'dft': (default) Density functional theory (DFT) for quantum chemical method

  • 'hf': Hartree-Fock method

mult

(default: 1) Spin multiplicity to be used in the QM calculation

scftype

Allowed values:

  • 'rhf': (default) restricted HF (or DFT) for singlet systems of molecular orbital calculation

  • 'uhf': unrestricted HF for higher spins

  • 'rohf': restricted open-shell

  • 'rks', 'uks', or 'roks': may also be specified, with identical behaviour

SCF options

damping

Allowed values:

  • True: (default) Use damping during SCF iterations

  • False: Do not use damping

diis

Allowed values:

  • True: (default) Use DIIS during SCF iterations

  • False: Do not use DIIS

direct

Allowed values:

  • False: (default) Switch off direct

  • True: Request direct evaluation for a given wavefunction.

guess

(default: '') Free text input for initial guess: can be either Python multi-line string (such as '''...''') or a filename (e.g., 'my_guess.txt') of plain text containing multi-line string.

maxiter

(default: 100) Maximum number of SCF cycles allowed

path

(default: '') Path to the executable file for a system call

pseudopotential

(default: '') Pseudopotential

Allowed types of value:

  • A Python str for pseudopotential name

  • A Python dict of element—to—pseudopotential mapping

restart

Allowed values:

  • False: (default) Not to restart from a saved wavefunction guess

  • True: Restart from a saved wavefunction guess

scf

(default: 1.0E-5) Threshold for SCF convergence