10.3.1 Interface to the NBO Package‣ 10.3 Orbital Analysis ‣ Chapter 10 Molecular Properties and Analysis ‣ Q-Chem 6.2 User’s Manual

Q-Chem incorporates v. 5 of the Natural Bond Orbital (NBO) package for molecular properties and wave function analysis, ⁴²⁷ Glendening E. D., Landis C. R., Weinhold F.
Wiley Interdiscip. Rev.: Comput. Mol. Sci.
(2012), 2, pp. 1. Link ^, ¹³⁴² Weinhold F., Landis C. R., Glendening E. D.
Int. Rev. Phys. Chem.
(2016), 35, pp. 399. Link and can interface to v. 6 ⁴²⁷ Glendening E. D., Landis C. R., Weinhold F.
Wiley Interdiscip. Rev.: Comput. Mol. Sci.
(2012), 2, pp. 1. Link and v. 7 ⁴²⁸ Glendening E. D., Landis C. R., Weinhold F.
J. Comput. Chem.
(2019), 40, pp. 2234. Link of the NBO program as well. The NBO5 package is invoked simply by setting NBO = TRUE in the $rem section and is initiated after the SCF wave function is obtained. If switched on for a geometry optimization, the NBO5 program will only be invoked at the end of the last optimization step.

To use either NBO6 or NBO7, the desired version of the NBO program must be purchased, downloaded, and installed separately from nbo.chem.wisc.edu. Q-Chem can then interface to an external version of the NBO program by: (a) setting the NBOEXE environment variable appropriately, and (b) setting both NBO = TRUE and NBO_EXTERNAL = TRUE in the $rem section of the Q-Chem input file. Note that support for (and new downloads of) the NBO6 program were discontinued in 2020. Existing copies should still interface to Q-Chem, and the newer NBO7 program should interface in the same way. The older NBO5 program continues to be a part of Q-Chem and the latter is what runs if NBO = TRUE is set without specifying NBO_EXTERNAL.

Users should consult the NBO User’s Manual for options and details relating to NBO calculations. 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. for details and suggestions.

NBO

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 10.9 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    0.000000    0.000000    0.523383
   O    0.000000    0.000000   -0.671856
   H    0.931138    0.000000    1.117280
   H   -0.931138    0.000000    1.117280
$end

$rem
   METHOD           pbe0
   BASIS            def2-sv(p)
   NBO              1
   PRINT_ORBITALS   true
   MOLDEN_FORMAT    true
$end

$nbo
   PLOT
$end

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10.3.1 Interface to the NBO Package