NWChem

NWChem is a computational chemistry code based on gaussian basis functions or plane-waves.

Setup

You first need to install a working copy of NWChem for ASE to call; follow the instructions on the NWChem website.

The default command that ASE will use to start NWChem is nwchem PREFIX.nwi > PREFIX.nwo. You can change this command by setting the environment variable ASE_NWCHEM_COMMAND. (For example, add a line to your .bashrc with export ASE_NWCHEM_COMMAND="my new command".)

The default command will only allow you to run NWChem on a single core. To run on multiple processors you will need to specify your MPI (or similar) command, which typically requires telling the MPI command the number of tasks to dedicate to the process. An example command to allow multiprocessing is mpirun -n $SLURM_NTASKS nwchem PREFIX.nwi > PREFIX.nwo, for the SLURM queueing system. If you use a different queueing system replace $SLURM_NTASKS with the appropriate variable, such as $PBS_NP.

Examples

Here is a command line example of how to optimize the geometry of a water molecule using the PBE density functional:

$ ase build H2O | ase run nwchem -p xc=PBE -f 0.02
Running: H2O
LBFGS:   0  09:58:54    -2064.914841       1.9673
LBFGS:   1  09:58:55    -2064.976691       0.1723
LBFGS:   2  09:58:55    -2064.977120       0.0642
LBFGS:   3  09:58:55    -2064.977363       0.0495
LBFGS:   4  09:58:56    -2064.977446       0.0233
LBFGS:   5  09:58:56    -2064.977460       0.0059
$ ase gui H2O.traj@-1 -tg "a(1,0,2),d(0,1)"
102.591620591 1.00793234388

An example of creating an NWChem calculator in the python interface is:

from ase.calculators.nwchem import NWChem

calc = NWChem(label='calc/nwchem',
              dft=dict(maxiter=2000,
                       xc='B3LYP'),
              basis='6-31+G*')

Parameters

The NWChem calculator represents nested keyword blocks in the input file using nested Python dictionaries. For example, consider the following block of input:

memory 1024mb

dft
  xc B3LYP
  mult 2
  odft
  convergence energy 1e-5 density 1e-4 gradient 5e-3
end

This would be represented by the following keyword arguments to the NWChem calculator:

NWChem(memory='1024mb',
       dft=dict(xc='B3LYP',
                mult=2,
                odft=None,
                convergence=dict(energy=1e-5,
                                 density=1e-4,
                                 gradient=5e-3),
                ),
       )

Most input files can be constructed in this way. The NWChem calculator also has several special keywords which do not directly enter into the input file; these are described in the table below

keyword type default value description
label str 'nwchem' Used to name input and output files. Also, used as the default name of the perm and scratch directory, unless otherwise specified.
theory str See description Theory specifies the kind of calculation you want to do. Currently supported values are dft, scf, mp2, ccsd, tce, tddft, pspw, band, and paw. Other settings may work, but have not been tested. If not provided, the Calculator will attempt to guess based on the provided keywords.
center bool False Whether NWChem should automatically center your atoms. You probably don’t want to change this.
autosym bool False Whether NWChem should automatically symmetrize your atoms. You probably don’t want to change this.
autoz bool False Whether NWChem should automatically generate a Z-matrix for your system.
basis str 3-21G If provided with a string, the specified basis set will be used for all atoms. Alternatively, you can set element-specific basis sets by passing a dict, e.g. basis=dict(C='6-31G', O='3-21G').
basispar str '' Additional keywords to go in the first line of the basis block.
task str See description What kind of calculation is to be performed, e.g. 'energy', 'gradient', or 'optimize'. If not provided, it will be automatically determined by the Calculator.
symmetry str '' The symmetry group or number of your system.
geompar str '' Additional keywords to go in the first line of the geometry block.
set dict dict() A set of keys and values to be added directly to the NWChem rtdb. This isn’t necessary for most commonly done tasks, but it is required for certain functionality in plane-wave mode.
class ase.calculators.nwchem.NWChem(restart=None, ignore_bad_restart_file=False, label='nwchem', atoms=None, command=None, **kwargs)[source]

NWChem keywords are specified using (potentially nested) dictionaries. Consider the following input file block:

>>> dft
>>>   odft
>>>   mult 2
>>>   convergence energy 1e-9 density 1e-7 gradient 5e-6
>>> end

This can be generated by the NWChem calculator by using the following settings:

>>> calc = NWChem(dft={'odft': None,
>>>                    'mult': 2,
>>>                    'convergence': {'energy': 1e-9,
>>>                                    'density': 1e-7,
>>>                                    'gradient': 5e-6,
>>>                                    },
>>>                    },
>>>               )

In addition, the calculator supports several special keywords:

theory: str
Which NWChem module should be used to calculate the energies and forces. Supported values are 'dft', 'scf', 'mp2', 'ccsd', 'tce', 'tddft', 'pspw', 'band', and 'paw'. If not provided, the calculator will attempt to guess which theory to use based on the keywords provided by the user.
xc: str
The exchange-correlation functional to use. Only relevant for DFT calculations.
task: str
What type of calculation is to be performed, e.g. 'energy', 'gradient', 'optimize', etc. When using 'SocketIOCalculator', task should be set to 'optimize'. In most other circumstances, task should not be set manually.
basis: str or dict

Which basis set to use for gaussian-type orbital calculations. Set to a string to use the same basis for all elements. To use a different basis for different elements, provide a dict of the form:

>>> calc = NWChem(...,
>>>               basis={'O': '3-21G',
>>>                      'Si': '6-31g'})
basispar: str
Additional keywords to put in the NWChem basis block, e.g. 'rel' for relativistic bases.
symmetry: int or str
The point group (for gaussian-type orbital calculations) or space group (for plane-wave calculations) of the system. Supports both group names (e.g. 'c2v', 'Fm3m') and numbers (e.g. 225).
autosym: bool
Whether NWChem should automatically determine the symmetry of the structure (defaults to False).
center: bool
Whether NWChem should automatically center the structure (defaults to False). Enable at your own risk.
autoz: bool
Whether NWChem should automatically construct a Z-matrix for your molecular system (defaults to False).
geompar: str
Additional keywords to put in the NWChem \(geometry\) block, e.g. 'nucleus finite' for gaussian-shaped nuclear charges. Do not set 'autosym', 'center', or 'autoz' in this way; instead, use the appropriate keyword described above for these settings.
set: dict

Used to manually create or modify entries in the NWChem rtdb. For example, the following settings enable pseudopotential filtering for plane-wave calculations:

>>> set nwpw:kbpp_ray .true.
>>> set nwpw:kbpp_filter .true.

These settings are generated by the NWChem calculator by passing the arguments:

>>> calc = NWChem(...,
>>>               set={'nwpw:kbpp_ray': True,
>>>                    'nwpw:kbpp_filter': True})
kpts: (int, int, int), or dict
Indicates which k-point mesh to use. Supported syntax is similar to that of GPAW. Implies theory='band'.
bandpath: BandPath object
The band path to use for a band structure calculation. Implies theory='band'.