Dear Q-Chem users,
Is there a specific keyword that is required while performing constrained (spatially fixed/frozen atoms) optimization [DFT-based] in both the ground and the excited-state?
I end up exhausting the maximum optimization (energy seems to converge, I have given the energy of the last few optimization cycle below) cycle. I tried the following steps:
- Tweaking the geometries (changing the dihedrals of flexible portions of the molecule such as the aliphatic chain-substituents of aromatic ring) and reoptimize the structure.
- Reduce the default tolerance of the optimization parameters to a smaller value using “THRESH” keyword. I have also played around with specific keywords such as ( GEOM_OPT_TOL_GRADIENT , GEOM_OPT_TOL_ENERGY etc) but no success!
- I have also tried lengthening the geom_opt_cycles from default 50 to 400 cycles.
My rem-section had this
BASIS = cc-pVDZ
JOB_TYPE = Optimization
MEM_TOTAL = 120000
METHOD = wB97XD
SCF_CONVERGENCE = 6 ! reduced_convergence_criteria_for_thresh=9
SYMMETRY_IGNORE = 1
GEOM_OPT_MAX_CYCLES = 400
THRESH = 9 ! reduced_geom_convergence_criteria
(Note: Thresh (and reduced scf convergence criteria) was used only after the default parameters didn’t give us ground state converged geometry)
91 XYZ !HisC-spatially _fixed_atom
Yet no success in the optimization of even the ground state of the molecule.
Energies of the last few optimization cycles:
Optimization parameters print at the last optimization cycle (400th) with Q-Chem reduced tolerance values for the optimization parameters:
Maximum Tolerance Cnvgd?
Gradient 0.001014 0.000300 NO
Displacement 0.053426 0.001200 NO
Energy change -0.000018 0.000001 NO
Its a fairly larger system with 112 atoms (tetrapyrrole ring with 2 neighboring 5-membered aromatic rings through hydrogen bonding interaction). The molecule is a closed-shell, +1 charged.
I was wondering if I am missing any constrained-optimization-specific keywords? I would also be happy to hear any suggestions you might have to get an optimized geometry with the constraints.