Complex basis function (CBF) methods for describing meta-stable electronic states (see Section 4.9.5) require the user to specify a basis set in which the most diffuse functions have a complex-scaled exponent. The real-valued basis functions are specified using the standard $rem keyword BASIS. The complex basis set is specified using the $rem keyword COMPLEX_BASIS, and it should be a superset of the real basis. If the basis is specified as general or mixed, the user-defined basis should be entered in an an additional $zbasis input section in the same format as the $basis section.
Q-Chem will automatically determine the additional basis functions in the complex basis and scales the exponents of those functions. For example, if BASIS is specified as “cc-pVTZ" and COMPLEX_BASIS is specified as “aug-cc-pVTZ," the augmenting functions will have complex-scaled exponents. The scaling is done according to
where is the exponent of a particular basis function and is specified by the $rem keyword COMPLEX_THETA.