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How can I calculate the dispersion energy using Gaussian 09? Where is it printed in the output file when using wb97xd/cc-pvdz sp? If it isn't printed by default, what flags can be used to turn printing on?

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    $\begingroup$ Depending on the functional you use, it either contains some dispersion correction or not. For those which do not you can manually add it with EmpiricalDispersion=... if it is parameterized. If not, you can specify the parameters via the command line. See the manual for detail. The omegaB97XD DFA already contains the dispersion; it is parametrized that way. You will not be able to find a value for the dispersion correction. $\endgroup$ Commented Dec 22, 2016 at 11:49

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The value of the empirical dispersion correction is printed when the print level is turned up using #p in the route line. For any method that contains an empirical dispersion correction, search for R6Disp or Dispersion energy, where other parts of the line will vary depending on the type of correction (-D2, -D3, or -D3(BJ)):

R6Disp:  Grimme-D2 Dispersion energy=       -0.0174233223 Hartrees.

This line is not printed with # or #n at the start of the route section.

For wavefunction methods that include dispersion via electron correlation, the dispersion contribution is non-separable. For a method like B2PLYP-D3, the printed "Dispersion energy" is the empirical correction, not the MP2 contribution!

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    $\begingroup$ This is very good to know and I have to admit I did not know about it. It is one of the neverending shortcomings of the Gaussian manual. $\endgroup$ Commented Jan 19, 2017 at 9:03

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