Evaluation of Diffuse Basis Sets for Simulations of Strong Field Ionization Using Time-Dependent Configuration Interaction with a Complex Absorbing Potential

For simulations of strong field ionization using time-dependent configuration with a complex absorbing potential (TDCI-CAP), standard molecular basis set must be augmented by several sets of diffuse functions to support the wave function as it is distorted by the strong field and interacts with the absorbing potential. Various sets of diffuse functions used in previous studies have been extended and evaluated for their ability to model the angular dependence of the strong field ionization. These sets include diffuse <i>s</i>, <i>p</i>, <i>d</i>, and <i>f</i> Gaussian functions with selected even-tempered exponents of the form 0.0001 × 2<i>ⁿ</i> placed on each atom. For single-centered test cases, the largest contribution to the ionization rate is from functions with a maximum in the radial distribution close to the onset of the complex absorbing potential, while functions with smaller exponents also contributed to the rate. For molecules, diffuse functions on adjacent centers overlap strongly, leading to linear dependencies. The transformation to remove these linear dependencies mixes functions of different angular momenta making it difficult to assess the importance of individual <i>s</i>, <i>p</i>, <i>d</i>, and <i>f</i> functions in simulating the rate for molecules. As an alternative, a hierarchy of diffuse basis sets was constructed by starting with a small set and adding one or two functions at a time. These basis sets were evaluated for their ability to reproduce the rate and shape of the angular dependence of strong field ionization. When combined with the aug-cc-pVTZ molecular basis set and an absorbing potential starting at 3.5 times the van der Waals radius for each atom, the most diffuse <i>s</i>, <i>p</i>, <i>d</i>, and <i>f</i> functions need to have exponents of 0.0032, 0.0032, 0.0064, and 0.0064, respectively, or smaller. Strong field ionization from electronegative atoms such as oxygen required additional <i>f</i> functions with tight exponents of 0.0512 and 0.1024. Diffuse basis sets that perform well for the angular dependence of the ionization rate with a static field are equally effective for strong field ionization with a linearly polarized 7-cycle 800 nm pulse.