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# 8.4.1 Introduction

(December 20, 2021)

Users may, on occasion, prefer to use non-standard basis, and it is possible to declare user-defined basis sets in Q-Chem input (see Chapter 3 on Q-Chem inputs). The format for inserting a non-standard user-defined basis set is both logical and flexible, and is described in detail in the job control section below.

Note that the SAD guess is not currently supported with non-standard or user-defined basis sets. The current default is to use SCF_GUESS = AUTOSAD, unless a mixed basis is specified. When using a mixed basis, the simplest alternative is to specify the GWH or CORE options for SCF_GUESS, but these are relatively ineffective other than for small basis sets. The recommended alternative is to employ basis set projection by specifying a standard basis set for the BASIS2 keyword. See Section 4.4 on initial guesses for more information.