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# 8.16.1 Introduction

(February 4, 2022)

The remainder of this Chapter consists of a brief reference guide to Q-Chem’s built-in ECPs. The ECPs vary in their complexity and their accuracy and the purpose of the guide is to enable the user quickly and easily to decide which ECP to use in a planned calculation.

The following information is provided for each ECP:

• The elements for which the ECP is available in Q-Chem. This is shown on a schematic Periodic Table by shading all the elements that are not supported.

• The literature reference for each element for which the ECP is available in Q-Chem.

• The matching orbital basis set that Q-Chem will use for light (i.e.. non-ECP atoms). For example, if the user requests SRSC ECPs—which are defined only for atoms beyond argon—Q-Chem will use the 6-311G basis set for all atoms up to Ar.

• The core electrons that are replaced by the ECP. For example, in the fit-LANL2DZ ECP for the Fe atom, the core is [Ne], indicating that the $1s$, $2s$ and $2p$ electrons are removed.

• The maximum spherical harmonic projection operator that is used for each element. This often, but not always, corresponds to the maximum orbital angular momentum of the core electrons that have been replaced by the ECP. For example, in the fit-LANL2DZ ECP for the Fe atom, the maximum projector is of $P$-type.

• The number of valence basis functions of each angular momentum type that are present in the matching orbital basis set. For example, in the matching basis for the fit-LANL2DZ ECP for the Fe atom, there the three $s$ shells, three $p$ shells and two $d$ shells. This basis is therefore almost of triple-split valence quality.