13. COSMO

COSMO is the continuum solvation 'Conductor-Like Screening' Model of A. Klamt and G. Schuurmann to describe dielectric screening effects in solvents.

1. A. Klamt and G. Schuurmann, J.Chem.Soc. Perkin Trans. 2, 1993, p799-805.

The NWChem COSMO module implements algorithm for calculation of the energy for Hartree-Fock (RHF and ROHF) and Kohn-Sham (DFT and UDFT) wavefunctions. At the present gradients are calculated by finite difference of the energy and the code does not work with spherical basis functions or ECPs. In the current implementation the code calculates the gas-phase energy of the system followed by the solution-phase energy, and returns the electrostatic contribution to the solvation free energy. The codes does not calculate the non-electrostatic contributions to the free energy.

Invoking the COSMO solvation model is done by specifying the input COSMO input block with the input options as follows:

cosmo
    dielec  <real dielec default 78.4>
    radius  <real atom1>
            <real atom2>
       . . .
            <real atomN>
    rsolv   <real rsolv default 0.00>
    iscren  <integer iscren default 0>
    minbem  <integer minbem default 2>
    maxbem  <integer maxbem default 3>
    ificos  <integer ificos default 0>
    lineq   <integer lineq default 1>
END

Dielec is the value of the dielectric constant of the medium, with a default value of 78.4 (the dielectric constant for water).

Radius is an array that specifies the radius of the spheres associated with each atom and that make up the molecule-shaped cavity. Default values are Van der Waals radii. Values are in units of angstroms. The codes uses the following Van der Waals radii by default:

      data vdwr(103) /
     1   0.80,0.49,0.00,0.00,0.00,1.65,1.55,1.50,1.50,0.00,
     2   2.30,1.70,2.05,2.10,1.85,1.80,1.80,0.00,2.80,2.75,
     3   0.00,0.00,1.20,0.00,0.00,0.00,2.70,0.00,0.00,0.00,
     4   0.00,0.00,0.00,1.90,1.90,0.00,0.00,0.00,0.00,1.55,
     5   0.00,1.64,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,
     6   0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,
     7   0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,
     8   0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,
     9   0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,
     1   0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,1.65,
     2   0.00,0.00,0.00/

with 0.0 values replaced by 1.80 . Other radii can be used as well. See for examples:

1. E. V. Stefanovich and T. N. Truong, Chem. Phys. Lett. 244 (1995) p65-74.
2. V. Barone, M. Cossi, and J. Tomasi, J. Chem. Phys. 107 (1997) p3210-3221.

Rsolv is a parameter used to define the solvent accessible surface. See the original reference of Klamt and Schuurmann for a description. The default value is 0.00 (in angstroms).

Iscren is a flag to define the dielectric charge scaling option. "iscren 1" implies the original scaling from Klamt and Schuurmann, mainly "(eps-1)/(eps+1/2)", where 'eps' is the dielectric constant. "iscren 0" implies the modified scaling suggested by Stefanovich and Truong, mainly "(eps-1)/eps". Default is to use the modified scaling. For high dielectric the difference between the scaling is not significant.

The next three parameters define the tesselation of the unit sphere. The approach follows the original proposal by Klamt and Schuurmann. A very fine tesselation is generated from maxbem refining passes starting from either an octahedron or an icosahedron. The boundary elements created with the fine tesselation are condensed down to a coarser tesselation based on minbem. The induced point charges from the polarization of the medium are assigned to the centers of the coarser tesselation. Default values are "minbem 2" and "maxbem 3". The flag ificos serves to select the original tesselation, "ificos 0" for an octahedron (default) and "ificos 1" for an icoshedron. Starting from an icosahedron yields a somewhat finer tesselation that converges somewhat faster. Solvation energies are not really sensitive to this choice for sufficiently fine tesselations.

The lineq parameter serves to select the numerical algorithm to solve the linear equations yielding the effective charges that represent the polarization of the medium. "lineq 0" selects an iterative method (default), "lineq 1" selects a dense matrix linear equation solver. For large molecules where the number of effective charges is large, the codes selects the iterative method.

The following example is for a water molecule in 'water', using the HF/6-31G** level of theory:

start
echo
 title "h2o"
geometry
o                  .0000000000         .0000000000        -.0486020332
h                  .7545655371         .0000000000         .5243010666
h                 -.7545655371         .0000000000         .5243010666
end
basis segment cartesian
  o library 6-31g**
  h library 6-31g**
end
cosmo
  dielec 78.0
  radius 1.40
         1.16
         1.16
  rsolv  0.50
  lineq  0
end
task scf energy