Origin off warping in the E ⊗ e Jahn-Teller problem:: Quadratic vibronic coupling versus anharmonicity and application to NaCl:Rh2+ and triangular molecules -: art. no. 184117

A general method is suggested which allows one to separate the quadratic, cubic, and pseudo Jahn-Teller contributions to the warping of the adiabatic potential energy surface (APES) of the E circle times e problem employing ab initio calculations. Numerical results were obtained for NaCl:Rh2+ in a cluster approximation and triangular molecules (Na-3,K-3,Cu-3,Ag-3) using density functional theory (DFT) and multireference second-order perturbation theory (CASPT2) methods. A largely unexpected result is that the contribution of cubic anharmonicity to the energy barrier between the minima of the lower and upper branches of the APES is dominant in all the systems and amounts for not less than 60% of the total. Another feature is that the three different contributions mentioned above may have different signs, thus either enhancing or diminishing each other, affecting in different ways the lower and upper branches of the APES. Other details of the numerical results are also analyzed.