657. More on charges in nwchem and gaussian

A now ten-year old paper introduced the concept of Pauling bond-strength conserving terminations (PBS ) in the use of molecular codes for calculations involving extended crystalline systems ('Quantum-Chemical Calculations of Carbon-Isotope Fractionation in CO2(g), Aqueous Carbonate Species, and Carbonate Minerals' by James R. Rustad, Sierra L. Nelmes, Virgil E. Jackson, and David A. Dixon --  see link). The authors used NWChem for the calculations, most likely due to the affiliation between the lead author and PNNL, where NWChem is developed, and where the researchers have been banned from using Gaussian.

I use Gaussian almost exclusively these days, mainly due to how fast it is.

Unfortunately, Gaussian and NWChem behave quite differently when it comes to introduction of specified nuclear charges, so I here compare the two codes in terms of how to set up PBS calculations.

NWChem (6.8):

scratch_dir /scratch
Title "charge"

Start  charge

echo

charge 0

geometry noautosym noautoz units angstrom
 Mg     0.00000     0.00000     0.00000
 O     0.00000     2.09000     0.00000
 O     1.47785     2.22045e-16     1.47785
 O     -1.47785     -1.11022e-16     1.47785
 O     0.00000     -2.09000     0.00000
 O     -1.47785     2.22045e-16     -1.47785
 O     1.47785     -1.11022e-16     -1.47785
 H1     -0.691981     2.65500     -0.691981 charge 0.5 
 H1     0.691981     2.65500     0.691981 charge 0.5 
 H1     1.87737     0.978609     1.87737 charge 0.5 
 H1     1.87737     -0.978609     1.87737 charge 0.5 
 H     -1.18539     7.33956e-09     2.56935
 H     -2.56935     -7.33957e-09     1.18539
 H     -0.691981     -2.65500     0.691981
 H     0.691981     -2.65500     -0.691981
 H     -1.87737     -0.978609     -1.87737
 H     -1.87737     0.978609     -1.87737
 H     1.18539     -2.20187e-08     -2.56935
 H     2.56935     2.20187e-08     -1.18539
end

basis "ao basis" spherical print
  H library "def2-svp"
  Mg library "def2-svp"
  O library "def2-svp"
END

dft
  mult 1
  direct
  XC pbe0
  grid xfine
  mulliken
end

task dft energy   

This gives an energy of -655.860806066326.

Removing the charges for H1 and setting the total charge to +2 gives an energy of -657.044328628867

Gaussian (16.A01):
WRONG:

%nprocshared=6
%Mem=800000000
%Chk=charge.chk
#P GFINPUT rPBE1PBE/def2svp 5D  NoSymm  Punch=(MO) Pop=(full) 

charge

0 1 ! charge and multiplicity
 Mg     0.00000     0.00000     0.00000
 O     0.00000     2.09000     0.00000
 O     1.47785     2.22045e-16     1.47785
 O     -1.47785     -1.11022e-16     1.47785
 O     0.00000     -2.09000     0.00000
 O     -1.47785     2.22045e-16     -1.47785
 O     1.47785     -1.11022e-16     -1.47785
 H(znuc=0.5)     -0.691981     2.65500     -0.691981
 H(znuc=0.5)     0.691981     2.65500     0.691981
 H(znuc=0.5)     1.87737     0.978609     1.87737
 H(znuc=0.5)     1.87737     -0.978609     1.87737
 H     -1.18539     7.33956e-09     2.56935
 H     -2.56935     -7.33957e-09     1.18539
 H     -0.691981     -2.65500     0.691981
 H     0.691981     -2.65500     -0.691981
 H     -1.87737     -0.978609     -1.87737
 H     -1.87737     0.978609     -1.87737
 H     1.18539     -2.20187e-08     -2.56935
 H     2.56935     2.20187e-08     -1.18539

gives an energy of -655.679686484!

However,

2 1  ! charge and multiplicity

gives an energy of -655.860712881, which is what we want.

Removing the znuc specifications and using

2 1  ! charge and multiplicity

gives an energy of -657.044229333

Keeping the znuc specifications and defining those protons as fragment 2, and the rest of the cluster as fragment 1

 2 1 -2 1 4 1! charge and multiplicity

gives an energy of -655.860712881


Conclusion: 
both NWChem and Gaussian can be made to use PBS, but while you use the intended cluster charge (0) in NWChem, you need to use the unmodified charge (+2) in gaussian.