Evaluation of the basis set superposition error (BSSE) can be automated in Q-Chem. To calculate BSSE-corrected binding energies, specify fragments in the $molecule section and set JOBTYPE to BSSE. The BSSE jobs are not limited to the SCF energies and can be evaluated for multi-fragment systems at any level of theory. Q-Chem separates the system into fragments as specified in the $molecule section and performs a series of jobs on (a) each isolated fragment, (b) each isolated fragment with the remaining atoms in the system replaced by the ghost atoms, and (c) the entire system. Q-Chem saves all calculated energies and prints out the uncorrected and the counterpoise (CP)-corrected interaction energies. The $rem_frgm section can be used to control calculations on fragments, however, make sure that the fragments and the entire system are treated equally. It means that all numerical methods and convergence thresholds that affect the final energies (such as SCF_CONVERGENCE, THRESH, PURECART, XC_GRID) should be the same for the fragments and for the entire system. Avoid using $rem_frgm in the BSSE jobs unless absolutely necessary.
Example 12.5 Evaluation of counterpoise-corrected intermolecular interaction energy for the water trimer at the (i) B3LYP-D3(BJ)/6-31+G(d), (ii) MP2/6-31++G(d,p) levels
$molecule 0 1 -- 0 1 O -0.089523 0.063946 0.086866 H 0.864783 0.058339 0.103755 H -0.329829 0.979459 0.078369 -- 0 1 O 2.632273 -0.313504 -0.750376 H 3.268182 -0.937310 -0.431464 H 2.184198 -0.753305 -1.469059 -- 0 1 O 0.475471 -1.428200 -2.307836 H -0.011373 -0.970411 -1.626285 H 0.151826 -2.317118 -2.289289 $end $rem JOBTYPE BSSE METHOD B3LYP DFT_D D3_BJ BASIS 6-31+G(d) SCF_CONVERGENCE 8 integral_symmetry FALSE point_group_symmetry False $end @@@ $molecule read $end $rem JOBTYPE BSSE METHOD MP2 BASIS 6-31++G(d,p) SCF_CONVERGENCE 8 integral_symmetry FALSE point_group_symmetry False $end