Vibrational frequencies are computed following the 1999 Gaussian white paper. Briefly, the Hessian matrix is converted into mass-weighted Cartesian coordinates, and translational and rotational motion is projected out using the Eckart–Sayvetz conditions. Vibrational modes and frequencies are computed as the eigenvectors and eigenvalues of the resulting Hessian matrix.
Thermochemical values are computed using standard expressions, generally following the 2000 Gaussian white paper. Symmetry-based corrections are applied using symmetry numbers obtained from Pymsym. To account for translation- and rotation-like small vibrational modes, Rowan applies the recommended Cramer–Truhlar correction, where all non-transition state modes below 100 cm−1 are raised to 100 cm−1 for the purpose of computing the entropic correction.