**************
Charge Fitting
**************

.. py:class:: ChargeFitting

Charge fitting calculations can be carried out using the :py:obj:`ChargeFitting()` driver, which
expects a QM-level :ref:`Theory <theories>` object, e.g.::

  my_frag = Fragment(coords='my_frag.xyz')

  my_theory = GAMESS_UK(frag=my_frag,
                        method='dft',
                        functional='b3lyp',
                        basis='6-31g*')

  my_fit1 = ChargeFitting(theory=my_theory,
                          npoints=2400,
                          type='shell',
                          vdw_scale=1.5,
                          nlayers=2,
                          vdw_incr=0.2)

The charge fitting calculation is run with::

  my_job.run(dryrun=False)

Then the resulting charges will be stored in :py:obj:`my_frag.charges` (see: `\<class Fragment\>`_).

.. _`\<class Fragment\>`: fragment.html


Methods
=======

.. py:method:: ChargeFitting.run()

Options
=======

The single-point driver takes the following arguments:

.. py:attribute:: theory

   Choose the theory level by specifying a :ref:`Theory <theories>` instance

.. py:attribute:: alpha

   (default: ``1.0``) Search direction

.. py:attribute:: a

   (default: ``0.0005``) Constant :py:attr:`a` (in (Ha/*e*)**2)

.. py:attribute:: b

   (default: ``0.2``) Constant :py:attr:`b` (in elementary charge *e*)

.. py:attribute:: conserve

   (default: ``10``) Conserves the total charge every :py:attr:`conserve` cycles

.. py:attribute:: constraints
   :noindex:

   (default: ``[]``) Constraint definition

.. py:attribute:: frozen

   (default: ``[]``) A list of indices of atoms to freeze

.. py:attribute:: initial

   (default: ``float('nan')``) Initial values of charges

.. py:attribute:: maxcycles
   :noindex:

   (default: ``10000``) Maximum cylces of iterations

.. py:attribute:: maxinners

   (default: ``10``) Maximum cycles of inner iterations 

.. py:attribute:: method
   :noindex:

   Allowed values:
   * ``'resp'``: (default) Kolllman's restrained electrostatic potential (RESP). See: C. I. Bayly, et al., J. Phys. Chem. 1993, 97, 10269

.. py:attribute:: nlayers

   (default: ``4``) Number of layers of grid points

.. py:attribute:: npoints

   (default: :py:attr:`200*theory.frag.natoms`) Number of grid points to generate

.. py:attribute:: tol_inner

   (default: ``1.0E-5``) Tolerance of convergence for inner iterations

.. py:attribute:: tolerance
   :noindex:

   (default: ``1.0E-12``) Tolerance of convergence 

.. py:attribute:: type

   Allowed values:
   * ``'shell'``: (default) Uses shell-like grid points

.. py:attribute:: vdw_scale

   (default: ``1.2``) van der Waals scale 

.. py:attribute:: vdw_incr

   (default: ``0.2``) Increment of van der Waals : :py:attr:`0.4/nlayers**0.5`