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I calculate the atomic charge using restricted electrostatic potential (RESP) and compute with the Hartree Fock method. This method is OK for organic compounds, but for complexes that contain transition metals, I need to use keyword Pop=(ReadRadii,MK) and read the radius of the metal in the input.

How can I know the correct radius for my transition metal? I looked on Google, and they always use 2.0 for copper or other atoms, etc.

Also, is there anyway where I can set a charge (charge that I want) on a specific atom in the Gaussian input?

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    $\begingroup$ As for the second question, you cannot - but you can set the total charge. The HF method (as well as DFT and all other ab initio methods) not only does not require you to set charge on specific atoms, but actually does not enable you to do so at all. The atomic charges are always automatically distributed given only the total charge as the constraint. $\endgroup$
    – wzkchem5
    Apr 3, 2022 at 15:14
  • $\begingroup$ @wzkchem5 . Thanks for your response. You were right. I am going to redistribute the charged manually after it is calculated by Gaussian. $\endgroup$ Apr 3, 2022 at 17:24
  • $\begingroup$ Since you have qmmm as a tag, the MM part can have partial charges defined $\endgroup$
    – B. Kelly
    Apr 4, 2022 at 2:25

1 Answer 1

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The radius (specifically the van der Waals radius) of the atoms is used during the Merz-Kollman procedure to determine what set of points should be sampled to get a reasonably accurate fit of the ESP. For some metals, the van der Waals radii can simply be looked up in a table. If you are unable to find radii for a particular element, an alternative would be to use the keyword mkuff. This uses radii computed with the Universal Force Field, which are available for every element in the periodic table.

As for specifying the charge, in principle you can apply almost any constraint on the charges during the fitting to the ESP. In practice, Gaussian allows you to set the charge before the fitting by including the pop=uncharged keyword, which only fits any initially unset charges. A simple example might look like, where I'm considering $\ce{H3O+}$ surrounded by point charges that are taking the place of surrounding water molecules:

#p HF/3-21g nosym pop=(esp,uncharged)

H3O+

1 1
O       0 -0.50964188 1.00550963 0.00000000
H       0 0.45035812 1.00550963 0.00000000
H       0 -0.83009647 1.91044546 0.00000000
H       0 -1.06370126 0.22153373 0.00000000
O-#6--0.6465214       0 -4.03581272 -0.34435261 0.00000000
H-#6-0.5119446       0 -3.07581272 -0.34435261 0.00000000
H-#6-0.4903441       0 -4.35626731 0.56058322 0.00000000
O-#6--1.0214505       0 2.38292014 2.90633604 0.00000000
H-#6-0.3316260       0 3.34292014 2.90633604 0.00000000
H-#6-0.2799662       0 2.06246555 3.81127188 0.00000000
O-#6--0.9783521       0 -3.70523421 3.56749306 0.00000000
H-#6-0.1651149       0 -2.74523421 3.56749306 0.00000000
H-#6-0.3022461       0 -4.02568879 4.47242889 0.00000000
O-#6--0.7575957       0 2.13498625 -0.56473828 0.00000000
H-#6-0.4518826       0 3.09498625 -0.56473828 0.00000000
H-#6-0.1925964       0 1.81453167 0.34019755 0.00000000
O-#6--0.5969358       0 -0.60425725 1.07007829 -1.97552048
H-#6-0.5439027       0 0.35574275 1.07007829 -1.97552048
H-#6-0.2754568       0 -0.92471184 1.97501412 -1.97552048
O-#6--0.6915599       0 -0.76708769 1.14464887 2.43407203
H-#6-0.4910393       0 0.19291231 1.14464887 2.43407203
H-#6-0.2999107       0 -1.08754227 2.04958470 2.43407203

You specify the MM atom type and charge directly after each element.

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