04 May 2012

135. Oniom in gaussian -- with a little bit of help from gromacs and openbabel

Example -- I want to do explicit solvent modelling of methanol in water. This is obviously an articifical approach, but generally applicable.

This is a rough approach to doing oniom calculations using gaussian 09

1. Pre-optimisation
Draw methanol and set up a simple calc using e.g. ecce to pre-optimise the structure with e.g. an implicit solvent model. Here's g03.g03in:

  1 %nprocshared=4
  2 %Chk=g09_oniom.chk
  3 #P rb3lyp/GEN 5D Opt=()  Freq=()  Punch=(MO) Pop=() scrf=(pcm,solvent=water)
  4 
  5 g09_oniom
  6 
  7 0 1 ! charge and multiplicity
  8  C     0.00000     0.00000     0.00000
  9  H     -0.675500     -0.675500     0.675500
 10  H     0.675500     -0.675500     -0.675500
 11  H     -0.675500     0.675500     -0.675500
 12  O     0.866025     0.866025     0.866025
 13  H     1.51843     0.213620     1.51843
 14 
 15  H  0
 16  S   3  1.00
 17       18.73113700      0.03349500
 18        2.82539400      0.23472700
 19        0.64012200      0.81375700
 20  S   1  1.00
 21        0.16127800      1.00000000
 22  ****
 23  O  0
 24  S   6  1.00
 25     5484.67170000      0.00183100
 26      825.23495000      0.01395000
 27      188.04696000      0.06844500
 28       52.96450000      0.23271400
 29       16.89757000      0.47019300
 30        5.79963500      0.35852100
 31  SP  3  1.00
 32       15.53961600     -0.11077800      0.07087400
 33        3.59993400     -0.14802600      0.33975300
 34        1.01376200      1.13076700      0.72715900
 35  SP  1  1.00
 36        0.27000600      1.00000000      1.00000000
 37  SP  1  1.00
 38        0.08450000      1.00000000      1.00000000
 39  D   1  1.00
 40        0.80000000      1.00000000
 41  ****
 42  C  0
 43  S   6  1.00
 44     3047.52490000      0.00183500
 45      457.36951000      0.01403700
 46      103.94869000      0.06884300
 47       29.21015500      0.23218400
 48        9.28666300      0.46794100
 49        3.16392700      0.36231200
 50  SP  3  1.00
 51        7.86827200     -0.11933200      0.06899900
 52        1.88128800     -0.16085400      0.31642400
 53        0.54424900      1.14345600      0.74430800
 54  SP  1  1.00
 55        0.16871400      1.00000000      1.00000000
 56  SP  1  1.00
 57        0.04380000      1.00000000      1.00000000
 58  D   1  1.00
 59        0.80000000      1.00000000
 60  ****

2. Solvation using gromacs
Take the output, g03.g03out, and use babel to export the optimised structure
babel -ig09 g03.g03out -oxyz molecule.xyz

The next few steps require gromacs:
editconf -f molecule.xyz -o molecule.gro -box 2 2 2
genbox -cp molecule.gro -cs spc216.gro -o solvated.gro
babel -igro solvated.gro -oxyz solvated.xyz

Because I'm lazy, I then add an extra column for high/low, chop off the first few lines, and add a column with zeros...because that works. Not sure what that's actually for. Molecule?

tail -n +3 solvated.xyz | gawk '{print $1,"0",$2,$3,$4,"Low"}'>oniom.xyz

Edit oniom.xyz by hand and change the lines with "Low" to "High" (or just H) for the atoms in the methanol molecule.

C 0 10.14000 10.34000 10.00000 High
H 0 9.65000 10.75000 10.90000 High
H 0 9.65000 10.75000 9.10000 High
H 0 11.19000 10.65000 10.00000 High
O 0 10.14000 8.91000 10.00000 High
H 0 9.22000 8.60000 10.00000 High
O 0 5.69000 12.75000 11.65000 Low
H 0 4.76000 12.68000 11.28000 Low
H 0 5.80000 13.64000 12.09000 Low
O 0 15.55000 15.11000 7.03000 Low
[..]
cp oniom.xyz oniom.in

Edit oniom.in and put in your gaussian instructions, e.g.:

%chk=methanol_explicit.chk
%mem=500MB
%nprocshared=3
#Oniom(rb3lyp/6-31+g*:uff) maxdisk=6000MB opt=()

Methanol in explicit water

0 1 0 1 0 1
C 0 10.14000 10.34000 10.00000 High
[...]
So, you specify Oniom(high level:low level) -- here dft and MM (via uff -- amber and charmm are available too -- but then you must carefully define the atom types. See here).

You don't always, but sometimes, have to specify the spin/multiplicity of the high and low level systems as well as the total spin and multiplicity. Anyway, you can: First the overall, the the high, then the low level. E.g. we have a chromium compound with an unpaired spin of 1 (multiplicity=2) and a charge of +3, and have a MM shell with five sodium atoms and a multiplicity of 1 (spin=0). The total charge is 8 and total multiplicity is 2. The line would read 8 2 3 2 0 1.

That's it! You're ready to roll.

The run takes about 3 minutes.

That's it!







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