History

2026.11.5.1 – Zero-point energies of symmetric and spherical tops

• Molecules with degenerate vibrations, symmetric tops such as NH3 and ethane and spherical tops such as CH4, are no longer refused: the step gives their anharmonic zero-point energy and harmonic frequencies. The anharmonic fundamentals and constants of degenerate modes are not available yet. Linear molecules are still refused.

• Validated against Gaussian 09 to about 1 cm^-1 by giving the molecules slightly asymmetric masses, where Gaussian uses the general formulas.

2026.11.5 – Initial release: anharmonic frequencies (VPT2) with any Model Chemistry

• Anharmonic fundamentals, overtones, combination bands and the anharmonic zero-point energy by second-order vibrational perturbation theory, with Fermi resonances treated variationally (GVPT2), for any program a Model Chemistry step chooses.

• Hessians or gradients at structures displaced along the normal modes, over the program’s warm MDI engine or as separate calculations in parallel, with tight convergence and a warning when the force constants are noisy.

• Validated against Gaussian 09’s and ORCA 6.1.1’s own VPT2. Molecules with degenerate vibrations (linear molecules, symmetric and spherical tops) are not handled yet.