Treatment of core electrons is controlled by N_FROZEN_CORE. Starting from version Q-Chem 5.0, the core electrons are frozen by default in most post-Hartree–Fock calculations. Selected virtual orbitals can also be frozen by using N_FROZEN_VIRTUAL keyword (the default for this is zero).
The number of core electrons in an atom is relatively well-defined, and consists of certain atomic shells. (Note that ECPs are available in both “small-core” and “large-core” varieties; see Chapter 8.9.) For example, in phosphorus the core consists of 1, 2, and 2 shells, for a total of ten electrons. In molecular systems, the core electrons are usually chosen as those occupying the lowest energy orbitals, where is the number of core electrons in the constituent atoms. In some cases, particularly in the lower parts of the periodic table, this definition is inappropriate and can lead to significant errors in the correlation energy. Vitaly Rassolov has implemented an alternative definition of core electrons within Q-Chem which is based on a Mulliken population analysis, and which addresses this problem.807
The current implementation is restricted to n-kl type basis sets such as 3-21 or 6-31, and related bases such as 6-31+G(d). There are essentially two cases to consider, the outermost 6G functions (or 3G in the case of the 3-21G basis set) for Na, Mg, K and Ca, and the 3d functions for the elements Ga—Kr. Whether or not these are treated as core or valence is determined by the CORE_CHARACTER $rem, as summarized in Table 6.1.
|CORE_CHARACTER||Outermost 6G (3G)||3d (Ga–Kr)|
|for Na, Mg, K, Ca|
Note: The default setting (N_FROZEN_CORE=FC) does not work correctly in QM/MM calculations. One should specify the number of frozen core orbitals explicitly.