The Parametric Molecular Electrostatic Potential of Wang and Ford [45,46] is generated. When `PMEPR` is used, extra data must follow the Z-matrix and symmetry data. This extra data defines the plane in which the electrostatic potential is plotted, and is summarized in the Table. Other keywords, e.g., `
MINMEP`(to print the minima) or `
PRTMEP`(to write data for `esplot` to use) *must* be present.

The various planes which can be drawn through the system are identified by the datum ICASE:

`ICASE=1`If`X0`is zero, then the plane generated passes through atoms`N1`,`N2`and`N3`. Obviously, these atoms should not lie on a straight line. If`X0`is non-zero, then the plane is parallel to the plane of atoms`N1`,`N2`and`N3`, but`X0`Ångstroms above it.`ICASE=2`If`X0`is zero, then the plane generated is perpendicular to the plane of`N1`,`N2`and`N3`, and includes the line from atom`N1`to`N2`. If`X0`is non-zero, then the plane is perpendicular to the plane of`N1`,`N2`and`N3`, and is`X0`Ångstroms above, and parallel to, the line from atom`N1`to`N2`.`ICASE=3`The plane used is that which bisects the angle`N2`-`N1`-`N3`.`X0`is meaningless in this context, and should be set to zero.

Examples of extra data:

To generate data for the plane through atoms 10, 11, and 20, use as extra data: "1 10 11 20 0" (do not include the quotation marks.) Alternatively, every datum can be expressed in keyword style, as "icase=1 n1=10 n2=11 n3=20 z0=0"