Understanding Numerical Gradients with TD-DFT

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2:39:31 PM PST - Wed, Mar 5th 2014

Hi all,

I am hoping to understand the output I am receiving over the course of a long calculation attempting to get excited state frequencies using TD-DFT and numerical gradients.

Here is the pertinent part of my input file:

dft
        mult 1
        xc xcamb88 1.00 lyp 0.81 vwn_5 0.19 hfexch 1.00
        cam 0.33 cam_alpha 0.19 cam_beta 0.46
        direct
        iterations 500
end

tddft
        RPA
        NROOTS  1
        NOTRIPLET
        TARGET  1
        MAXVECS 100
        FREEZE  core 81 virtual 247
end

raman
        normal
        lorentzian
        low     600
        high    1700
end

property
        response        1 0.070421
        damping         0.007
end

task tddft raman

What I expected would be similar to a ground state numerical calculation where each atom in the structure is "bumped" + and - in each x,y,z coordinate, with a subsequent SCF and TD for each "bump." However, for each "bump" in my calculation I am seeing 12 or so SCF/TD calculations being performed instead of the one I expected, significantly increasing my expected computational time. I would greatly appreciate any help in understanding how this calculation differs from the ground state numerical calculation of frequencies.

Thanks in advance!

-Paul

Here is an egrep of my output file on "atom: |step | DFT Module|TDDFT Module so you can see how many times each calculation is performed. Inbetween atom 6 xyz 1(-) and 6 xyz 2(+) there are 12 additional DFT/TDDFT calculations performed. Full output file can be found here[1].:

 atom:   6 xyz: 1(-) wall time:    7334.1      date:  Wed Feb 26 23:20:33 2014
                                 NWChem DFT Module
                                NWChem TDDFT Module
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step     1 energy       -2207.93256747
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step    -1 energy       -2207.93265797
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step     2 energy       -2207.93252298
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step    -2 energy       -2207.93268483
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step     3 energy       -2207.93263476
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step    -3 energy       -2207.93263476
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step     4 energy       -2207.93266191
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step    -4 energy       -2207.93256311
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step     5 energy       -2207.93268551
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step    -5 energy       -2207.93252228
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step     6 energy       -2207.93263476
                                 NWChem DFT Module
                                NWChem TDDFT Module
 step    -6 energy       -2207.93263476
 atom:   6 xyz: 2(+) wall time:   14011.8      date:  Thu Feb 27 01:11:51 2014