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PBE POTENTIAL repeat(pbe-potential-options) [NODIelectric] [TOLerance real] [RTOLerance real] pbe-potential-options ::= [MAXITS int] [HISTsize int] [RADIUS real] [problem-option] problem-option ::= {LINEar } {NONLinear}

The `PBE POTENTIAL` command invokes the Gauss-Siedel PBE solver, and
the above options affects how the solution is performed. If the
`PARALLEL LOOPS` command is used, see Parallel Command, the
potential will be calculated in parallel across available CPU's on a
Silicon Graphic multiprocessor workstation. Parallel efficiency is high.
N.B., for large memory jobs running on SGI workstations, it is
critically important to set the `STACK` resource limit down from the
default value. See Rlimit Command, for more information.

After the potential is calculated, the PBE energy per atom is calculated and stored in an array for this purpose.

The options have the following meaning:

- Option
- Purpose
`NODIELECTRIC`-
If this option is specified, the potential is calculated assuming
the dielectric is equal to the solvent value, keyword
`SOLVENT`in PBE SETUP Command. The Poisson-Boltzmann equation is not used, but rather the potential contribution from each atom is summed onto each grid point. The boundary is also modified. `TOLERANCE`-
If the maximum change in any potential value during a cycle over all the
grid points is less
than the
`TOLERANCE`option, then the solver terminates. Values smaller than 10^-6 are generally too small to be achieved. The default setting is 0.001. `RTOLERANCE`-
If the RMS of the changes in potential value during a cycle over all the
grid points (maximum residuals) is less than this option, the solver terminates. This option
is less restrictive than the
`TOLERANCE`option above, and therefore, terminates sooner. The maximum change is typically an order of magnitude bigger than the RMS change. The default setting is 0.0, in which case, the test is not used. `MAXITS`-
The specifies the maximum number of iterations allowed for the solver.
Because of the odd-even checkboard cycling, two iterations are necessary to cover
all the grid points. The default is 100.
`HISTSIZE`-
When the PBE solver is working toward a small tolerance, it is possible
for a potentially infinite loop to be entered. In order to detect this,
CONGEN maintains an array of maximum residuals for twice the number of
cycles given by
`HISTSIZE`. At the end of each iteration over all the atoms, the minimum value for the maximum residual for the last`HISTSIZE`cycles is compared against the minimum for the previous`HISTSIZE`cycles. If it has not improved, then the PBE solver terminates. Effectively, this mechanism checks for any improvement in the solution of`HISTSIZE`cycles. The default value is 30. `RADIUS`-
Overrides the default setting, see Introduction to Poisson-Boltzmann Electrostatics, for the spectral radius. If this
value is set to zero, then overrelaxation is not used. If the value is
negative, then the absolute value is used as the overrelaxation
parameter.
`LINEAR``NONLINEAR`-
These two options control whether the linearized form of the Poisson-Boltzmann
equation is solved. Generally, only the linearized form should be used.
The non-linear form has a modest radius of convergence, and large charges will
result in divergence of the solver. When the non-linear form is used, execution
time is much slower, and overrelaxation is turned off. The default is
`LINEAR`.