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,
gives an energy of -655.860712881, which is what we want.2 1 ! charge and multiplicity
Removing the znuc specifications and using
gives an energy of -657.0442293332 1 ! charge and multiplicity
Keeping the znuc specifications and defining those protons as fragment 2, and the rest of the cluster as fragment 1
gives an energy of -655.8607128812 1 -2 1 4 1! charge and multiplicity
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.