Ab Initio Complex Potential Energy Surfaces From Standard Quantum Chemistry Packages

Abstract

Situations in which a molecule in a given configuration is electronically bound while in another configuration is autoionized are widespread in nature. In these situations, the change in molecular configuration due to nuclear dynamics is the reason the molecule emits free electrons to the surrounding, ie, autoionizes. Such a situation may even happen to molecules in their ground electronic state, for example, it can happen to H2−: at some bond lengths, the molecule is autoionized, at some bond lengths its ground state is bound, and at sufficiently large internuclear distances a stable hydrogen atom and a stable negative charged hydrogen, H−, in their ground electronic states, are obtained. In addition, such situations can be seen in electronic scattering from molecules and in cold molecular collisions. For example, in a collision between electronically excited helium atom and a hydrogen molecule in its ground state, metastable complex He*–H2 is formed. As time passes this complex decays to helium in its ground state, H2+, and a free electron. In all these cases the molecular dynamics play a key role as the molecules are autoionized. This poses a problem, since the Born–Oppenheimer (BO) approximation is applicable only when the decay process due to ionization is ignored. Therefore, in order to study molecular dynamics and take autoionization into consideration, one should calculate the potential energy surfaces (PES) by imposing outgoing boundary conditions (OBCs) on the electronic wavefunctions. Doing so, the electronic molecular spectrum will be discrete (no continuum), where the PES will be either real (bound electronic states) or complex (metastable molecules that ionize). These complex potential energy surfaces (CPES) are what enables one to take into consideration the electronic autoionization in the molecular dynamics. Nevertheless, calculating CPES by standard quantum chemistry packages (SQCPs) is not a trivial task, since they were designed to calculate bound electronic excited states. Bound states lie on the real plane, unlike metastable states (resonances); therefore, explicit calculation of resonances requires modification of SQCPs. Several different possibilities for calculating CPES by modifying SQCPs are discussed in this review. Yet, the holy grail is to be able to use SQCPs, which are highly efficient codes, for calculating resonances without changing the codes. The main focus of this review will be on new methods, we have developed, that enable calculating CPESs from SQCPs, ie, without any modifications of standard codes. Such methods allow the calculations of polyatomic CPESs, as indicated by our preliminary results.