As discussed above, results obtained various types of PCMs are quite sensitive
to the details of the cavity construction. Q-Chem’s implementation of PCMs,
using Lebedev grids, simplifies this construction somewhat, but leaves the
radii of the atomic spheres as empirical parameters (albeit ones for which
widely-used default values are provided). An alternative implementation of the
SS(V)PE solvation model is also available,^{171} which attempts
to further eliminate empiricism associated with cavity construction by taking
the cavity surface to be a specified iso-contour of the solute’s electron
density. [We call this the isodensity implementation of SS(V)PE in
Table 12.3, and it is based on Chipman’s “symmetrized” form
of the $\mathbf{K}$ matrix.^{171, 521}] In this case, the
cavity surface is discretized by projecting a single-center Lebedev grid onto
the iso-contour surface. Unlike the PCM implementation discussed in
Section 12.2.2, for which point-group symmetry is disabled, this
implementation of SS(V)PE supports full symmetry for all Abelian point groups.
The larger and/or the less spherical the solute molecule is, the more points
are needed to get satisfactory precision in the results. Further experience
will be required to develop detailed recommendations for this parameter.
Values as small as 110 points are usually sufficient for diatomic or triatomic
molecules. The default value of 1202 points is adequate to converge the energy
within 0.1 kcal/mol for solutes the size of mono-substituted benzenes.

Energy gradients are also not available for this implementation of SS(V)PE, although they are available for the implementation described in Section 12.2.2 in which the cavity is constructed from atom-centered spheres. As with the PCMs discussed in that section, the solute may be described using Hartree-Fock theory or DFT; post-Hartree–Fock correlated wave functions can also take advantage of molecular orbitals that are polarized using SS(V)PE. Researchers who use the isodensity SS(V)PE feature are asked to cite Ref. 173.

In related work, Pomogaeva and
Chipman^{734, 735, 736, 737}
recently introduced a “composite method for implicit representation of
solvent” (CMIRS) that is based on SS(V)PE electrostatics but adds
non-electrostatic terms. This model is available in Q-Chem^{1025} and
is discussed in Section 12.2.6. In its current
implementation, CMIRS requires an isodensity SS(V)PE calculation, However, the
current implementation computes the non-electrostatic interactions using the
cavity and the solute’s charge density generated from the isodensity SS(V)PE.
To use the CMIRS model, an isodensity SS(V)PE calculation must be requested (as
described below), and the IDEFESR must be set to 1 in the *$svp* input section (see Section 12.2.5.2).

An isodensity SS(V)PE calculation is requested by setting
SOLVENT_METHOD = ISOSVP in the *$rem* section, in addition
to normal job control variables for a single-point energy calculation. Whereas
the other solvation models described in this chapter use specialized input
sections (*e.g.*, *$pcm*) in lieu of a slew of *$rem* variables, the isodensity
SS(V)PE code is an interface between Q-Chem and a code written by
Chipman,^{171} so some *$rem* variables are used for
job control of isodensity SS(V)PE calculations. These are listed below.

SVP_MEMORY

Specifies the amount of memory for use by the solvation module.

TYPE:

INTEGER

DEFAULT:

125

OPTIONS:

$n$ corresponds to the amount of memory in MB.

RECOMMENDATION:

The default should be fine for medium size molecules
with the default Lebedev grid, only increase if needed.

SVP_PATH

Specifies whether to run a gas phase computation prior to performing the
solvation procedure.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
runs a gas-phase calculation and after
convergence runs the SS(V)PE computation.
1
does not run a gas-phase calculation.

RECOMMENDATION:

Running the gas-phase calculation provides a good guess to start the solvation
stage and provides a more complete set of solvated properties.

SVP_CHARGE_CONV

Determines the convergence value for the charges on the cavity. When the
change in charges fall below this value, if the electron density is converged,
then the calculation is considered converged.

TYPE:

INTEGER

DEFAULT:

7

OPTIONS:

$n$
Convergence threshold set to ${10}^{-n}$.

RECOMMENDATION:

The default value unless convergence problems arise.

SVP_CAVITY_CONV

Determines the convergence value of the iterative isodensity cavity procedure.

TYPE:

INTEGER

DEFAULT:

10

OPTIONS:

$n$
Convergence threshold set to ${10}^{-n}$.

RECOMMENDATION:

The default value unless convergence problems arise.

SVP_GUESS

Specifies how and if the solvation module will use a given guess for the
charges and cavity points.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
No guessing.
1
Read a guess from a previous Q-Chem solvation computation.
2
Use a guess specified by the *$svpirf* section from the input

RECOMMENDATION:

It is helpful to also set SCF_GUESS to READ when using a guess
from a previous Q-Chem run.

This last *$rem* variable requires specification of a *$svpirf* input section, the format
for which is the following:

$svpirf <# point> <x point> <y point> <z point> <charge> <grid weight> <# point> <x normal> <y normal> <z normal> $end

More refined control over SS(V)PE jobs is obtained using a *$svp* input section.
These are read directly by Chipman’s SS(V)PE solvation module
and therefore must be specified in the context of a FORTRAN namelist. The
format is as follows:

$svp <KEYWORD>=<VALUE>, <KEYWORD>=<VALUE>,... <KEYWORD>=<VALUE> $end

For example, the section may look like this:

$svp RHOISO=0.001, DIELST=78.39, NPTLEB=110 $end

The following keywords are supported in the *$svp* section:

DIELST

The static dielectric constant.

INPUT SECTION: *$svp*

TYPE:

FLOAT

DEFAULT:

78.39

OPTIONS:

real number specifying the constant.

RECOMMENDATION:

The default value 78.39 is appropriate for water solvent.

IDEFESR

Specifies whether to request a CMIRS calculation.

INPUT SECTION: *$svp*

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
do not invoke a CMIRS calculation.
1
do invoke a CMIRS calculation.

RECOMMENDATION:

ISHAPE

A flag to set the shape of the cavity surface.

INPUT SECTION: *$svp*

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
use the electronic isodensity surface.
1
use a spherical cavity surface.

RECOMMENDATION:

Use the default surface.

RHOISO

Value of the electronic isodensity contour used to specify the cavity surface.
(Only relevant for ISHAPE = 0.)

INPUT SECTION: *$svp*

TYPE:

FLOAT

DEFAULT:

0.001

OPTIONS:

Real number specifying the density in electrons/bohr${}^{3}$.

RECOMMENDATION:

The default value is optimal for most situations. Increasing the value
produces a smaller cavity which ordinarily increases the magnitude of the
solvation energy.

RADSPH

Sphere radius used to specify the cavity surface (Only relevant for ISHAPE=1.)

INPUT SECTION: *$svp*

TYPE:

FLOAT

DEFAULT:

Half the distance between the outermost atoms plus 1.4 Ångstroms.

OPTIONS:

Real number specifying the radius in bohr (if positive) or in Ångstroms (if negative).

RECOMMENDATION:

Make sure that the cavity radius is larger than the length of the molecule.

INTCAV

A flag to select the surface integration method.

INPUT SECTION: *$svp*

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
Single center Lebedev integration.
1
Single center spherical polar integration.

RECOMMENDATION:

The Lebedev integration is by far the more efficient.

NPTLEB

The number of points used in the Lebedev grid for the single-center surface
integration. (Only relevant if INTCAV = 0.)

INPUT SECTION: *$svp*

TYPE:

INTEGER

DEFAULT:

1202

OPTIONS:

Valid choices are:
6, 18, 26, 38, 50, 86, 110, 146, 170, 194, 302, 350, 434, 590, 770,
974, 1202, 1454, 1730, 2030, 2354, 2702, 3074, 3470, 3890, 4334,
4802, or 5294.

RECOMMENDATION:

The default value has been found adequate to obtain the energy to within 0.1
kcal/mol for solutes the size of mono-substituted benzenes.

NPTTHE, NPTPHI

The number of ($\theta $,$\varphi $) points used for single-centered surface
integration (relevant only if INTCAV = 1.)

INPUT SECTION: *$svp*

TYPE:

INTEGER

DEFAULT:

8,16

OPTIONS:

$\theta $,$\varphi $ specifying the number of points.

RECOMMENDATION:

These should be multiples of 2 and 4 respectively, to provide symmetry
sufficient for all Abelian point groups. Defaults are too small for all but
the tiniest and simplest solutes.

LINEQ

Flag to select the method for solving the linear equations that determine the
apparent point charges on the cavity surface.

INPUT SECTION: *$svp*

TYPE:

INTEGER

DEFAULT:

1

OPTIONS:

0
use LU decomposition in memory if space permits, else switch to LINEQ = 2
1
use conjugate gradient iterations in memory if space permits, else use LINEQ = 2
2
use conjugate gradient iterations with the system matrix stored externally on disk.

RECOMMENDATION:

The default should be sufficient in most cases.

CVGLIN

Convergence criterion for solving linear equations by the conjugate gradient
iterative method (relevant if LINEQ = 1 or 2.)

INPUT SECTION: *$svp*

TYPE:

FLOAT

DEFAULT:

1.0E-7

OPTIONS:

Real number specifying the actual criterion.

RECOMMENDATION:

The default value should be used unless convergence problems arise.

Note that the single-center surface integration approach that is used to find the isodensity surface may fail for certain very non-spherical solute molecules. The program will automatically check for this, aborting with a warning message if necessary. The single-center approach succeeds only for what is called a “star surface”, meaning that an observer sitting at the center has an unobstructed view of the entire surface. Said another way, for a star surface any ray emanating out from the center will pass through the surface only once. Some cases of failure may be fixed by simply moving to a new center with the ITRNGR parameter described below. But some surfaces are inherently non-star surfaces and cannot be treated with this program until more sophisticated surface integration approaches are developed and implemented.

ITRNGR

Translation of the cavity surface integration grid.

INPUT SECTION: *$svp*

TYPE:

INTEGER

DEFAULT:

2

OPTIONS:

0
No translation (*i.e.*, center of the cavity at the origin
of the atomic coordinate system)
1
Translate to the center of nuclear mass.
2
Translate to the center of nuclear charge.
3
Translate to the midpoint of the outermost atoms.
4
Translate to midpoint of the outermost non-hydrogen atoms.
5
Translate to user-specified coordinates in Bohr.
6
Translate to user-specified coordinates in Ångstroms.

RECOMMENDATION:

The default value is recommended unless the single-center integrations
procedure fails.

TRANX, TRANY, TRANZ

$x$, $y$, and $z$ value of user-specified translation (only relevant if
ITRNGR is set to 5 or 6).

INPUT SECTION: *$svp*

TYPE:

FLOAT

DEFAULT:

0, 0, 0

OPTIONS:

$x$, $y$, and $z$ relative to the origin in the appropriate units.

RECOMMENDATION:

None.

IROTGR

Rotation of the cavity surface integration grid.

INPUT SECTION: *$svp*

TYPE:

INTEGER

DEFAULT:

2

OPTIONS:

0
No rotation.
1
Rotate initial $xyz$ axes of the integration grid to coincide
with principal moments of nuclear inertia (relevant if ITRNGR = 1)
2
Rotate initial $xyz$ axes of integration grid to coincide with
principal moments of nuclear charge (relevant if ITRNGR = 2)
3
Rotate initial $xyz$ axes of the integration grid through user-specified
Euler angles as defined by Wilson, Decius, and Cross.

RECOMMENDATION:

The default is recommended unless the knowledgeable user has good reason otherwise.

ROTTHE ROTPHI ROTCHI

Euler angles ($\theta $, $\varphi $, $\chi $) in degrees for user-specified
rotation of the cavity surface (relevant if IROTGR = 3).

INPUT SECTION: *$svp*

TYPE:

FLOAT

DEFAULT:

0,0,0

OPTIONS:

$\theta $, $\varphi $, $\chi $ in degrees

RECOMMENDATION:

None.

IOPPRD

Specifies the choice of system operator form.

INPUT SECTION: *$svp*

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
Symmetric form.
1
Non-symmetric form.

RECOMMENDATION:

The default uses more memory but is generally more efficient, we recommend its
use unless there is shortage of memory available.

By default, Q-Chem will check the validity of the single-center expansion by searching for the isodensity surface in two different ways: first, working inwards from a large distance, and next by working outwards from the origin. If the same result is obtained (within tolerances) using both procedures, then the cavity is accepted. If the two results do not agree, then the program exits with an error message indicating that the inner isodensity surface is found to be too far from the outer isodensity surface.

Some molecules, for example C${}_{60}$, can have a hole in the middle. Such molecules have two different “legal” isodensity surfaces, a small inner one inside the “hole”, and a large outer one that is the desired surface for solvation. In such cases, the cavity check described in the preceding paragraph causes the program to exit. To avoid this, one can consider turning off the cavity check that works out from the origin, leaving only the outer cavity determined by working in from large distances.

ICVICK

Specifies whether to perform cavity check

INPUT SECTION: *$svp*

TYPE:

INTEGER

DEFAULT:

1

OPTIONS:

0
no cavity check, use only the outer cavity
1
cavity check, generating both the inner and outer cavities and compare.

RECOMMENDATION:

Consider turning off cavity check only if the molecule has a hole and if a star
(outer) surface is expected.