Hi,
I am working with two molecule (OPP and TEA) and need to calculate SP energy for OPP*- and TEA+ system. I was using TEA optimized cation geom and OPP S1 anion geom for my calculation. But how can I define the charge separately in Qchem? Also, if I need to use specific distance between OPP*- and TEA+, what is the correct way to do that?
$molecule
0 1
N 0.0998431 -0.8344849 -0.6362965
C -1.2856633 -0.4235917 -0.8784963
C -1.7381546 0.7125031 0.0302687
C 0.6771913 -1.4870734 -1.8141758
C 1.1056524 -0.5011607 -2.8939234
C 0.1995745 -1.6909007 0.5484548
C 1.5893312 -1.6884771 1.1726260
H -1.9727794 -1.2836206 -0.7822318
H -1.3643899 -0.0865008 -1.9149273
H -1.7022036 0.4348554 1.0871491
H -2.7689379 0.9960307 -0.2010262
H -1.1001413 1.5894883 -0.1095992
H -0.0246198 -2.2303814 -2.2337453
H 1.5584485 -2.0477288 -1.4926920
H 0.2635276 0.0755791 -3.2855712
H 1.5594580 -1.0333838 -3.7348668
H 1.8404356 0.2036108 -2.4950963
H -0.1086066 -2.7254760 0.3116312
H -0.5121598 -1.3247506 1.2925606
H 2.3513658 -2.0681824 0.4866460
H 1.6046129 -2.3214080 2.0645027
H 1.8737775 -0.6738202 1.4643434
H 0.0542933 -1.8929925 -2.4348850
C 0.0119422 -0.9376911 -2.9421985
C -0.0331072 0.2767087 -2.1826826
C -0.0781566 1.4911085 -2.9421985
H -0.1205077 2.4464099 -2.4348850
C -0.0784285 1.4813119 -4.3200333
H -0.1159549 2.4238048 -4.8578519
C -0.0331072 0.2767087 -5.0308764
H -0.0331072 0.2767087 -6.1158865
C 0.0122141 -0.9278945 -4.3200333
H 0.0497405 -1.8703874 -4.8578519
C -0.0331072 0.2767087 -0.7608857
C -0.0343620 1.4953315 0.0139683
C -0.0318524 1.4953315 1.3722957
C -0.0331072 0.2767087 2.1471497
C -0.0343620 -0.9419141 1.3722957
C -0.0318524 -0.9419141 0.0139683
C -0.0331072 0.2767087 3.5689466
C 0.0119422 1.4911085 4.3284625
C 0.0122141 1.4813119 5.7062973
C -0.0331072 0.2767087 6.4171404
C -0.0784285 -0.9278945 5.7062973
C -0.0781566 -0.9376911 4.3284625
H -0.0312237 2.4520818 -0.4921539
H -0.0349907 2.4520818 1.8784179
H -0.0312237 -1.8986644 1.8784179
H -0.0349907 -1.8986644 -0.4921539
H 0.0542933 2.4464099 3.8211490
H 0.0497405 2.4238048 6.2441159
H -0.0331072 0.2767087 7.5021505
H -0.1159549 -1.8703874 6.2441159
H -0.1205077 -1.8929925 3.8211490
$end
I don’t quite understand the coordinates that you gave (some funny bond lengths, doesn’t seem like the noncovalent complex that I was expecting). Let me generalize. You are doing a calculation on noncovalent complex A…B and you want to look at the charge-separation ion pair A-…B+ instead? Assuming that’s what you mean, I would caution first of all that this is not necessarily well-defined, but there are a couple of ways to get at something like this.
- Use a superposition-of-fragments guess (SCF_GUESS = FRAGMO). Then you can specify charge and multiplicity separately for either fragment. Assuming both A and B are closed-shell singlets, this would look something like:
$molecule
0 1
--
-1 2
[coordinates for A(anion)]
--
1 2
[coordinates for B(cation)]
$end
This will use a guess that is (A-)…(B+). The caveat is that it’s just a guess; the relaxed SCF orbitals for the complex may transfer the charge back to B to give you neutral fragments. You can try to use MOM, STEP, or SGM algorithms to locate an orbital-relaxed SCF solution that looks like your initial guess. Those methods are described here:
https://manual.q-chem.com/latest/topic_mom.html
https://manual.q-chem.com/latest/sect_STEP-SCF.html
https://manual.q-chem.com/latest/sect_SGM-SCF.html
- If that still fails, then I think you’re left with constrained DFT, which will allow you to specify the charge on either fragment by fiat. CDFT is described here:
5.11 Methods Based on “Constrained” DFT‣ Chapter 5 Density Functional Theory ‣ Q-Chem 6.4 User’s Manual
Thanks for the suggestions. I plan to use the FRAMO method for my analysis. I have an additional question regarding how to place two molecules, A and B, at a specific separation. In particular, I would like the distance between the two molecules to be defined with respect to their centers of mass. What is the correct and recommended approach to construct such a geometry?
That’s a tricky one but if you have their relative orientation the way that you want it, then you can calculation c.o.m. of each and do this yourself in Cartesian coordinates.