xTB calculations are requested through the usual $rem section. The only mandatory keyword is METHOD = XTB; all other controls are optional. Charge and multiplicity are specified in the standard way in the $molecule section. In xTB mode, Q-Chem automatically uses valence electrons when constructing the occupation pattern, so users should simply provide the chemically correct molecular charge and multiplicity.
METHOD
METHOD
XTB
TYPE:
STRING
DEFAULT:
None
OPTIONS:
None.
RECOMMENDATION:
Set METHOD to XTB.
BASIS
BASIS
XTB
TYPE:
STRING
DEFAULT:
XTB (implied)
OPTIONS:
None
RECOMMENDATION:
For METHOD = XTB, BASIS = XTB is implied and may be omitted.
No other basis is allowed.
XTB_FERMI_TEMP
XTB_FERMI_TEMP
Controls the Fermi-smearing temperature used for fractional occupations in xTB calculations.
TYPE:
INTEGER
DEFAULT:
300
OPTIONS:
Temperature in Kelvin.
RECOMMENDATION:
The default is usually appropriate. Setting XTB_FERMI_TEMP = 0
recovers integer occupations.
Standard SCF controls such as SCF_ALGORITHM and SCF_CONVERGENCE are also available. SCF_ALGORITHM = DIIS is the default, while SCF_ALGORITHM = GDM or SCF_ALGORITHM = ROBUST can be useful for difficult cases.