I've been trying to model the Stokes shift of the first excited singlet of a small organic probe molecule in solvents.
The issue is that when modelling the molecule excited state in water, the geometry optimisation of the excited state is not well behaved. A minimum is shown after 2 steps (which is significantly higher than previous studies with same XC, solvent model and basis), and then the energy starts to oscillate and increase. How can I improve convergence of this calculation?
In vacuum, and for apolar COSMO-SMD models, the geometry optimisation has been well behaved. Convergence is achieved in around 8 optimisation steps.
I've tried multiple approaches. I've attempted with both COSMO and COSMO SMD. I've made the trust radius larger and smaller. I've used alternate starting geometries and restarts (such as the relaxed geometry of vacuum excited state). I've also checked for implemented patches in NWchem.
My initial guess is that the forces act to optimise the molecule and that when the new strongly interacting COSMO cavity is constructed it is energetically dis-favourable.
Here's an example input:
geometry units angstroms
---
end
basis
---
end
dft
XC B3LYP
iterations 50
decomp
end
TDDFT
RPA
NROOTS 10
PRINT HIGH
CIVECS
NOTRIPLET
TARGET 1
TARGETSYM a
GRAD
ROOT 1
END
END
DRIVER
MAXITER 50
XYZ MOL-B3LYP-6-311+gdp
END
COSMO
do_cosmo_smd true
solvent water
END
task TDDFT optimize
The corresponding example output :
@ Step Energy Delta E Gmax Grms Xrms Xmax Walltime
@ ---- ---------------- -------- -------- -------- -------- -------- --------
@ 0 -711.93801005 0.0D+00 0.03302 0.00519 0.00000 0.00000 3582.9
@ 1 -711.94026873 -2.3D-03 0.01163 0.00227 0.05829 0.18416 8252.7
@ 2 -711.94019608 7.3D-05 0.00748 0.00145 0.01870 0.04729 12887.2
@ 3 -711.94013249 6.4D-05 0.00606 0.00118 0.00770 0.02076 17597.9
@ 4 -711.94008912 4.3D-05 0.00554 0.00107 0.00365 0.01090 22224.8
@ 5 -711.94006818 2.1D-05 0.00524 0.00102 0.00245 0.00777 26823.9
@ 6 -711.94003572 3.2D-05 0.00499 0.00097 0.00285 0.00931 31527.7
@ 7 -711.94001458 2.1D-05 0.00473 0.00092 0.00281 0.00927 36081.6
@ 8 -711.93997759 3.7D-05 0.00450 0.00088 0.00336 0.01119 40650.6
@ 9 -711.93995431 2.3D-05 0.00431 0.00084 0.00308 0.01022 45185.9
@ 10 -711.93991563 3.9D-05 0.00408 0.00079 0.00365 0.01222 49788.4
@ 11 -711.93989370 2.2D-05 0.00391 0.00076 0.00296 0.00989 54345.7
@ 12 -711.93985394 4.0D-05 0.00370 0.00072 0.00411 0.01369 58954.7
@ 13 -711.93983413 2.0D-05 0.00357 0.00070 0.00274 0.00915 63516.5
@ 14 -711.93978598 4.8D-05 0.00336 0.00065 0.00470 0.01559 68200.9
@ 15 -711.93976709 1.9D-05 0.00325 0.00063 0.00258 0.00863 72740.6
@ 16 -711.93971708 5.0D-05 0.00307 0.00060 0.00471 0.01568 77396.3
@ 17 -711.93970510 1.2D-05 0.00297 0.00058 0.00242 0.00804 82011.4
Thank you for any help with this issue,
Duncan
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