Field-dependent ionisation potential by constrained density functional theory

Abstract

Constrained density functional theory is used in a novel approach to calculate the vertical ionisation potential in an electric field up to 30 MV/cm for water, ethane and ethene. Two density functionals, the Becke, three-parameter, Lee–Yang–Parr functional (B3LYP) and the B3LYP functional modified by the Coulomb-attenuating method (CAM-B3LYP), and two basis sets, cc-pVTZ and aug-cc-pVTZ are employed and compared with an earlier study using a point-charge approach. It is found that the field-dependent ionisation potential obtained from constrained density functional theory is in good agreement with the previous work. Furthermore, the results for the two density functionals are close to each other and the effect of augmentation of the basis set is small for the field-dependent ionisation potential. Limitations of the methodology at high electric fields are discussed. The method presented here demonstrates how constrained density functional theory can be used in general to study molecule–electron interactions.