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 â€śTHRESHâ€ť 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

$rem

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

$end

(Note: Thresh (and reduced scf convergence criteria) was used only after the default parameters didnâ€™t give us ground state converged geometry)

$opt

FIXED

91 XYZ !HisC-spatially _fixed_atom

ENDFIXED

$end

Yet no success in the optimization of even the ground state of the molecule.

Energies of the last few optimization cycles:

-2707.349765079

-2707.349777164

-2707.349791392

-2707.349806339

-2707.349822085

-2707.349830906

-2707.349844057

-2707.349859100

-2707.349882210

-2707.349903446

-2707.349927979

-2707.349951354

-2707.349967897

-2707.349987905

-2707.350004919

-2707.350022825

-2707.350037582

-2707.350050833

-2707.350067663

-2707.350084984

-2707.350103316

-2707.350123358

-2707.350141690

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.

Thank you,

Siva