Q-Chem can evaluate electrostatic potentials on a grid of points.
Electrostatic potential evaluation is controlled by the *$rem* variable ESP_GRID.

ESP_GRID

Controls evaluation of the electrostatic potential on a grid of points. If
enabled, the output is in an ASCII file, plot.esp, in the format $x,y,z,$ esp
for each point.

TYPE:

INTEGER

DEFAULT:

none no electrostatic potential evaluation

OPTIONS:

$-3$
same as the option $-1$ but in connection with STATE_ANALYSIS=TRUE.
This computes the ESP for all excited-state densities, transition densities,
and electron/hole densities.
$-2$
same as the option $-1$, plus evaluate the ESP of the *$external_charges*
$-1$
read grid input via the *$plots* section of the input deck
$0$
Generate the ESP values at all nuclear positions
+$n$
read $n$ grid points in bohr from the ASCII file ESPGrid

RECOMMENDATION:

None

The following example illustrates the evaluation of electrostatic potentials on a grid. Note that IANLTY must also be set to 200.

$molecule 0 1 H 0.0 0.0 0.35 H 0.0 0.0 -0.35 $end $rem METHOD hf BASIS 6-31g** IANLTY 200 ESP_GRID -1 $end $plots plot the HOMO and the LUMO on a line 1 0.0 0.0 1 0.0 0.0 15 -3.0 3.0 0 0 0 0 0 $end

We can also compute the electrostatic potential for the transition density, which can be used, for example, to compute the Coulomb coupling in excitation energy transfer.

ESP_TRANS

Controls the calculation of the electrostatic potential of the transition density

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

TRUE
compute the electrostatic potential of the excited state transition density
FALSE
compute the electrostatic potential of the excited state electronic density

RECOMMENDATION:

NONE

The electrostatic potential is a complicated object and it is not uncommon to
model it using a simplified representation based on atomic charges. For this
purpose it is well known that Mulliken charges perform very poorly. Several
definitions of ESP-derived atomic charges have been given in the literature,
however, most of them rely on a least-squares fitting of the ESP evaluated on a
selection of grid points. Although these grid points are usually chosen so
that the ESP is well modeled in the “chemically important” region, it still
remains that the calculated charges will change if the molecule is rotated.
Recently an efficient rotationally invariant algorithm was proposed that sought
to model the ESP not by direct fitting, but by fitting to the multipole
moments.^{Simmonett:2005} By doing so it was found that the fit to the ESP
was superior to methods that relied on direct fitting of the ESP. The
calculation requires the traceless form of the multipole moments and these are
also printed out during the course of the calculations. To request these
multipole-derived charges, set MM_CHARGES = TRUE in the *$rem* section.

MM_CHARGES

Requests the calculation of multipole-derived charges (MDCs).

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

TRUE
Calculates the MDCs and also the traceless form of the multipole moments

RECOMMENDATION:

Set to TRUE if MDCs or the traceless form of the multipole
moments are desired. The calculation does not take long.