Analytic energy third derivatives for paired-excited multiconfiguration self-consistent-field wave functions

A theoretical formalism and its implementation are described for the analytic evaluation of energy third derivatives for the paired-excited multiconfiguration self-consistent-field wave function. Cubic force constants and vibration–rotation interaction constants for H2O, CH2(3B1), CH2(1A1), HCN, and NH3 are reported for five different basis sets—from double zeta through triple zeta plus double polarization. These results are compared with those from single configuration SCF wave functions and with the available experimental data.