Finite-temperature vibrational full configuration interaction (vol 119, e1949503, 2021)

Thermodynamic functions of an ideal molecular gas due to its anharmonic vibrations are evaluated in a wide range of temperature (T) by the vibrational full-configuration-interaction (FCI) method using a quartic force field and a finite number (N) of harmonic-oscillator basis functions along each normal mode. The thermodynamic functions considered are the grand potential (Omega), internal energy (U), and entropy (S). They are compared with those obtained from the Bose-Einstein theory with or without truncation of the harmonic-oscillator basis functions after quantum number N-1. The comparison reveals that the finite-basis-set errors in Omega and U are, respectively, k(B)T ln (k(B)T/Nh omega) k(B)T per mode in the high-T limit, obscuring anharmonic effects when k(B)T > h omega, where omega is the lowest mode frequency. The benchmark data for several low-order perturbation corrections to Omega, U, and S are also obtained as the numerical derivatives of their FCI values with respect to dimensionless perturbation strength, and the domain of T and N in which these data are reliable (for the N -> infinity limits) is discussed. [GRAPHICS]