Properties can be calculated for both the Hartree-Fock and DFT wave functions. The properties that are available are:
- Natural bond analysis
- Dipole, quadrupole, and octupole moment
- Mulliken population analysis and bond order analysis
- Electrostatic potential (diamagnetic shielding) at nuclei
- Electric field and field gradient at nuclei
- Electron and spin density at nuclei
- NMR shielding (GIAO method)
- NMR hyperfine coupling (Fermi-Contact and Spin-Dipole expectation values)
- NMR indirect spin-spin coupling
The properties module is started when the task directive TASK <theory> property is defined in the user input file. The input format has the form:
PROPERTY [property keyword] [CENTER ((com || coc || origin || arb <real x y z>) default coc)] END
Most of the properties can be computed for Hartree-Fock (closed-shell RHF, open-shell ROHF, and open-shell UHF), and DFT (closed-shell and open-shell spin unrestricted) wavefunctions. The NMR chemical shift is limited to closed-shell wave functions, whereas the NMR hyperfine and indirect spin-spin coupling require a UHF or ODFT wave function.
Each property can be requested by defining one of the following keywords:
NBOFILE DIPOLE QUADRUPOLE OCTUPOLE MULLIKEN ESP EFIELD EFIELDGRAD ELECTRONDENSITY HYPERFINE SHIELDING [<integer> number_of_atoms <integer> atom_list] SPINSPIN [<integer> number_of_pairs <integer> pair_list] ALL
The "ALL" keyword generates all currently available properties.
Both the NMR shielding and spin-spin coupling have additional optional parameters that can be defined in the input. For the shielding the user can define the number of atoms for which the shielding tensor should be calculated, followed by the list of specific atom centers. In the case of spin-spin coupling the number of atom pairs, followed by the atom pairs, can be defined (i.e., spinspin 1 1 2 will calculate the coupling for one pair, and the coupling will be between atoms 1 and 2).
For both the NMR spin-spin and hyperfine coupling the isotope that has the highest abundance and has spin, will be chosen for each atom under consideration.
The user also has the option to choose the center of expansion for the dipole, quadrupole, and octupole calculations.
[CENTER ((com || coc || origin || arb <real x y z>) default coc)]
com is the center of mass, coc is the center of charge, origin is (0.0, 0.0, 0.0) and arb is any arbitrary point which must be accompanied by the coordinated to be used. Currently the x, y, and z coordinates must be given in the same units as UNITS in GEOMETRY.
Response calculations can be calculated as follows:
property response 1 7.73178E-2 # response order and frequency in hartree velocity # use modified velocity gauge for electric dipole end
Response calculations are currently supported only for order 1 (linear response), single frequency, electric field and mixed electric-magnetic field perturbations. The output consists of the electric polarizability and optical rotation tensors (alpha, beta for optical rotation) in atomic units. If the 'velocity' keyword is absent, the dipole-length form will be used for the dipole integrals. This is a bit faster. The isotropic optical rotation is origin independent when using the velocity gauge. Works with HF and density functionals for which linear response kernels are implemented in NWChem.
Please refer to the following papers for further details:
- J. Autschbach, Comp. Lett. 3, 131(2007)
- M. Krykunov, J. Autschbach, J. Chem. Phys. 123, 114103 (2005)
- J.R. Hammond, N. Govind, K. Kowalski, J. Autschbach, S.S. Xantheas, J. Chem. Phys. 131, 214103 (2009)
Raman calculations can be performed by specifying the Raman block. These calculations are performed in conjunction with polarizability calculations.
RAMAN [ (NORMAL | | RESONANCE) default NORMAL ] [ (LORENTZIAN | | GAUSSIAN) default LORENTZIAN ] [ LOW <double low default 0.0> ] [ HIGH <double high default ‘highest normal mode’> ] [ FIRST <integer first default ‘7’> ] [ LAST < integer last default ‘number of normal modes’ > ] [ WIDTH <double width default 20.0> ] [ DQ!<double dq default 0.01> ] END task dft raman
task dft raman numerical
Sample input block:
property response 1 8.8559E-2 damping 0.007 end raman normal lorentzian end
The keyword NBOFILE does not execute the Natural Bond Analysis code, but simply creates an input file to be used as input to the stand-alone NBO code. All other properties are calculated upon request.
Following the successful completion of an electronic structure calculation, a Natural Bond Orbital (NBO) analysis may be carried out by providing the keyword NBOFILE in the PROPERTY directive. NWChem will query the rtdb and construct an ASCII file, <file_prefix>.gen, that may be used as input to the stand alone version of the NBO program, gennbo. <file_prefix> is equal to string following the START directive. The input deck may be edited to provide additional options to the NBO calculation, (see the NBO user's manual for details.)
Users that have their own NBO version can compile and link the code into the NWChem software. See the INSTALL file in the source for details.