The atomic Radii

General comments

atomic radii are not well defined
most van der Waals radii refer back to an article by Bondi in 1964 doi:10.1021/j100785a001
UFF radii are defined in an article by Rappe in 1992 doi:10.1021/jp8111556
Batsnov compares radii in 2001 doi:10.1023/A:1011625728803
Truhlar has a redefinition based on the density in 2009 doi:10.1021/jp8111556
PCM and SMD use different radii, go to the end of this page to see more information
PCM do not have internal radii for some atoms, mainly TMs!

modify all radii

If you want to manually change ALL the VDW radii use the read command and specify an alpha, this changes all radii

# opt b3lyp/6-311g(d,p) int=ultrafine scf=conver=9
  geom=cartesian scrf(smd,solvent=water,read)

Title

-2 1
   6           0        0.000142    0.000521    0.000000
   8           0        0.007404   -1.299082    0.000000
   8           0        1.122188    0.655527    0.000000
   8           0       -1.129734    0.643021   -0.000000

alpha=1.15

without you will get:

 Atomic radii         : SMD-Coulomb.
 Cavity type          : VdW (van der Waals Surface) (Alpha=1.000).

using the above you will get:

 Atomic radii         : SMD-Coulomb.
 Cavity type          : Scaled VdW (van der Waals Surface) (Alpha=1.150).

modify radii definition

If you want to change the VDW radii definition used the read command and specify the type of radii, this changes all radii to the method identified

# opt b3lyp/6-311g(d,p) int=ultrafine scf=conver=9
  geom=cartesian scrf(pcm,solvent=water,read)

Title

-2 1
   6           0        0.000142    0.000521    0.000000
   8           0        0.007404   -1.299082    0.000000
   8           0        1.122188    0.655527    0.000000
   8           0       -1.129734    0.643021   -0.000000

Radii=Bondi

without you will get:

 Atomic radii         : UFF (Universal Force Field).
 Cavity type          : Scaled VdW (van der Waals Surface) (Alpha=1.100).

using the above you will get:

 Atomic radii         : Bondi.
 Cavity type          : Scaled VdW (van der Waals Surface) (Alpha=1.100).

modify specific radii

this currently does not work, the instructions on the gaussian web-site result in an error

ModifySph: Alters parameters for one or more spheres. The modified spheres 
can be indicated in the PCM input in lines following this keyword having
the following format:
   atom radius [alpha]
where atom is the atom number or element type.

include

ModifySph
    O   1.6000

and get

Using the following non-standard input for PCM:
 ModifySph
     O   1.6000
 ------------------------------------------------------------------------------
 The following wrong keyword(s) have been found:

 1.6000
 ------------------------------------------------------------------------------
 Error termination via Lnk1e in /Applications/g16/l301.exe at Sat Dec 23 16:41:53 2017.

The correct way to approach this is

use atom number and not the symbol (and not the atomic number!)
this option must be terminated with a blank line

Below the oxygen which happens to be the 4th atom in the z-matrix is given the radius 1.6 angstroms

ModifySph

    4   1.6000