Frequency calculations for a system with frozen atom constraints are invalid as they introduce imaginary frequencies. These arise because the Hessian computed for the frequency calculation has different physics compared to the gradient computed during the fixed-atom optimization.
One possible workaround is to zero out the elements of the Hessian associated with the frozen atoms. This removes the contribution of the frozen atoms to the normal modes and reduces the number of frequencies computed. For a system with atoms, where atoms are frozen, there will be normal modes, or if the system is linear.
FRZN_OPT
FRZN_OPT
Controls whether the job uses zeroed Hessian technique in the frequency calculations
TYPE:
LOGICAL
DEFAULT:
False
OPTIONS:
False
Do not use the zeroed out Hessian
True
Use the zeroed out Hessian
RECOMMENDATION:
False
FRZ_ATOMS
FRZ_ATOMS
Controls the number of frozen atoms
TYPE:
INTEGER
DEFAULT:
No default
OPTIONS:
User defined
RECOMMENDATION:
None
Example 9.7 Frequency calculations using zeroed out Hessian technique. Note the frozen atom indices are specified in the $frozen_opt section.
$molecule 0 1 C -0.0014570824 1.4001468208 0.0000000000 H -0.0030029737 2.4867609686 0.0000000000 C -1.2093554750 0.6986986619 0.0000000000 H -2.1525287416 1.2392252926 0.0000000000 C -1.2094237515 -0.6985901650 0.0000000000 H -2.1508487680 -1.2427180000 0.0000000000 C -0.0003285453 -1.3965954363 0.0000000000 H -0.0006460438 -2.4839940415 0.0000000000 C 1.2098250013 -0.6978846084 0.0000000000 H 2.1493830451 -1.2446769146 0.0000000000 C 1.2133329176 0.6987868208 0.0000000000 H 2.1550504173 1.2408406009 0.0000000000 $end $rem JOBTYPE freq METHOD b3lyp BASIS 6-31G* FRZN_OPT 1 ! Turns on the freq calculation with frozen Hessians FRZ_ATOMS 2 ! No. of frozen atoms SYM_IGNORE true SYMMETRY false $end $frozen_opt 1 11 $end