Q. Does the selection of the operators affect the smoothness of the potential surface?

 

A. To calculate large molecules, the SAC-CI program performs the perturbation selection of the excitation operators. The set of the selected operators depends on the geometry. This dependency can affect the smoothness of the potential surface, when the low-level threshold was used for the selection. We note that the calculated potential surface is rather smooth, and the vibrational frequency become very close to the experiments, when the high-level selection was performed.

   We recommend using GSUM (Group SUM of the excitation operators) method for calculating the potential surface. The GSUM method takes group sum of the operators selected at different geometries, and the final set of the excitation operators are qualitatively equivalent in all of the examined geometries. Please find some descriptions in the SAC-CI guide. We also provided a sample input for the GSUM calculations in test644.