9.12.1 Introduction

The study of metastable electronic states like temporary anions presents a major challenge for computational chemists. Finding, for example, a Hartree-Fock (HF) Self-Consistent Field (SCF) solution which describes the electronic state of a given temporary anion is usually an arduous and tricky task. That makes the prospect of performing a simple HF-based AIMD simulation of temporary anions even more daunting. On top of the inherent difficulties of the electronic structure problem, one also has to take into account the fact that upon its formation, the temporary anion is, in general, subject to two competing processes: electron autodetachment (i.e. AB${}^{-}\to$AB $+$ e${}^{-}$) and dissociative electron attachment (DEA) (i.e. AB${}^{-}\to$A $+$ B${}^{-}$). However, the need to be able to perform such an AIMD simulation cannot be overstated given that such an effort has the potential to offer very important insights into the mechanisms connecting the formation of temporary anions to their DEA products, for example.

Taking advantage of the Complex Absorbing Potential’s (CAP) analytic gradients already implemented in Q-Chem ⁹¹ Benda Z., Jagau T.-C.
J. Chem. Phys.
(2017), 146, pp. 031101. Link , we have combined the general principles of AIMD simulations with the CAP method (§7.10.9) to allow users to run AIMD simulations for temporary anions. Based on the Born-Oppenheimer approximation, the method, denoted ‘CAP-AIMD’, makes it possible to propagate the nuclei on a complex potential energy surface (CPES) computed on the fly .

Starting a CAP-AIMD simulation on the right CPES is paramount to a successful simulation. Failure to do so will lead to wrong results. For this reason, before moving on to discuss how one can run CAP-AIMDs with Q-Chem, we deem it fit to provide brief guidelines on how to find correct CAP-HF SCF solutions for temporary anions in §9.12.2.