For methanimine, the cubic and quartic force fields have also been evaluated by numerical differentiation from the molecular Hessians calculated for Cartesian displacements δx of 10–2 and 10–3 Å with Dalton2013 using the expressions41 42where E xixj, E xixjxk and E xixjxkxl represent, respectively, the second-, third- and fourth-order derivatives of the energy with respect to the Cartesian components in superscript, and using convergence thresholds of 10–8 for both the molecular orbital (MO) coefficients and relative to the norm of the perturbed MO coefficients when solving the response equations. The same convergence criteria have been applied to all fully analytic calculations. We remark that the errors in the calculated properties resulting from these strict thresholds are negligible.