Exploring Excited State Potential Surfaces: Nonadiabatic Couplings and Minimum-Energy Crossing Points

  • Nonadiabatic (or vibronic) coupling is the most important physical quantity governing transitions between potential energy surfaces, such as internal conversion and intersystem crossing.
  • The minimum-energy crossing point (MECP) in the seam space where multiple electronic states are degenerate are the most important points which connect photochemical reaction pathways.
  • Q-Chem offers options to calculate nonadiabatic coupling at the traditional CIS and TDDFT level, as well as the spin-flip (SF) CIS and TDDFT level. SF methods are the only methods which afford correct topology around conical intersections.
  • Q-Chem can locate MECPs without the need to evaluate nonadiabatic couplings, which are not available for many methods. Multiple algorithms are also available.

Calculated derivative coupling vectors for naphthalene


Calculated z-component of the derivative coupling between the 31\u03a3g+ and 51\u03a3g+ states of He2