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(May 16, 2021)

Q-Chem offers a number of standard and special customization features. One of the most important is that of supplying additional diffuse functions. Diffuse functions are often important for studying anions and excited states of molecules, and for the latter several sets of additional diffuse functions may be required. These extra diffuse functions can be generated from the standard diffuse functions by applying a scaling factor to the exponent of the original diffuse function. This yields a geometric series of exponents for the diffuse functions which includes the original standard functions along with more diffuse functions.

When using very large basis sets, especially those that include many diffuse
functions, or if the system being studied is very large, linear dependence in
the basis set may arise. This results in an over-complete description of the
space spanned by the basis functions, and can cause a loss of uniqueness in the
molecular orbital coefficients. Consequently, the SCF may be slow to converge
or behave erratically. Q-Chem will automatically check for linear dependence
in the basis set, and will project out the near-degeneracies, if they exist.
This will result in there being slightly fewer molecular orbitals than there are
basis functions. Q-Chem checks for linear dependence by considering the
eigenvalues of the overlap matrix. Very small eigenvalues are an indication
that the basis set is close to being linearly dependent. The size at which the
eigenvalues are considered to be too small is governed by the *$rem* variable
BASIS_LIN_DEP_THRESH. By default this is set to 6, corresponding to
a threshold of ${10}^{-6}$. This has been found to give reliable results;
however, if you have a poorly behaved SCF, and you suspect there maybe linear
dependence in your basis, the threshold should be increased. The smallest
overlap matrix eigenvalue is printed in the Q-Chem output file, and usually when
this number goes below ${10}^{-5}$, numerical issues caused by basis function linear
dependence may occur and the SCF calculation may not give reasonable solutions.
If the smallest overlap matrix eigenvalue is less than the
square root of the integral threshold, a warning message urging to tighten the
integral threshold (*e.g.*, setting THRESH = 14) will be printed out. In
any case, when a linear dependence issue is suspected, tightening the
integral threshold should be tried first.

PRINT_GENERAL_BASIS

Controls print out of built in basis sets in input format

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

TRUE
Print out standard basis set information
FALSE
Do not print out standard basis set information

RECOMMENDATION:

Useful for modification of standard basis sets.

BASIS_LIN_DEP_THRESH

Sets the threshold for determining linear dependence in the basis set

TYPE:

INTEGER

DEFAULT:

6
Corresponding to a threshold of ${10}^{-6}$

OPTIONS:

$n$
Sets the threshold to ${10}^{-n}$

RECOMMENDATION:

Set to 5 or smaller if you have a poorly behaved SCF and you suspect linear
dependence in you basis set. Lower values (larger thresholds) may affect the
accuracy of the calculation.