10 Molecular Properties and Analysis

10.3 Interface to the NBO Package

Q-Chem incorporates the Natural Bond Orbital package (v. 5.0 and 6.0) for molecular properties and wave function analysis. The NBO 5.0 package is invoked by setting the $rem variable NBO to TRUE and is initiated after the SCF wave function is obtained.

Note:  If switched on for a geometry optimization, the NBO package will only be invoked at the end of the last optimization step.

Users should consult the NBO User’s Manual for options and details relating to exploitation of the features offered in this package. The NBO 6.0 package306, 304 can be downloaded by the user from nbo6.chem.wisc.edu, and can be invoked by: (a) setting the NBOEXE environment variable, and (b) include both NBO = TRUE and RUN_NBO6 = TRUE in the Q-Chem input file.

NBO analysis is also available for excited states calculated using CIS or TDDFT. Excited-state NBO analysis is less well-developed, and users should be aware that the convergence of the NBO search procedure may be less well-behaved for excited states than it is for ground states. Excited state may require specification of additional NBO parameters in the $nbo section that is described below. Consult Ref. 1023 for details and suggestions.

NBO
       Controls the use of the NBO package.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Do not invoke the NBO package. 1 Do invoke the NBO package, for the ground state. 2 Invoke the NBO package for the ground state, and also each CIS, RPA, or TDDFT excited state.
RECOMMENDATION:
       None

The general format for passing options from Q-Chem to the NBO program is shown below:

$nbo
   {NBO program keywords, parameters and options}
$end

Note:  1. $rem variable NBO must be set to TRUE before the $nbo keyword is recognized. 2. Q-Chem does not support facets of the NBO package which require multiple job runs 3. Output of the NBOs can be triggered by the PRINT_ORBITALS and MOLDEN_FORMAT keywords. In this case two MOLDEN sections are written to outfile. The first one corresponds to the regular MOs, the second one to the NBOs. 4. Print-out of the full set of NAOs, NHOs, NBOs, and NLMOs can be triggered via the PLOT keyword in the $nbo section. The files of interest are FILE.31 – FILE.39 in $QCSCRATCH/savename. These files can be opened by, e.g., the ChemCraft and Jmol programs.

Example 10.4  Basic input for NBO computation on formaldehyde. The NBOs are printed to outfile in Molden format and the full set of files in native NBO format are written to $QCSCRATCH/savename.

$molecule
0 1
C  0        0  0.523383
O  0        0 -0.671856
H  0.931138 0  1.11728
H -0.931138 0  1.11728
$end

$rem
method = pbe0
basis = def2-sv(p)
nbo = 1
print_orbitals = true
molden_format = true
$end

$nbo
PLOT
$end