Q-Chem can evaluate electrostatic potentials on a grid of points. Electrostatic potential evaluation is controlled by the $rem variable ESP_GRID.
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 electrostatic potential 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.
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.1032 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.
The derivatives of electrostatic potential with respect to positions give electric fields, which is an important physical quantity for studies ranging from vibrational spectroscopy to catalysis. Similar to the case of ESP, Q-Chem can be used to compute the values of E-field on nuclear positions or a given grid, which is also controlled by the $rem variable ESP_GRID. The calculation of electric field is turned on when the value of ESP_EFIELD is greater than 0:
$molecule 0 1 O -0.9112629280 1.0922672019 1.0200719528 H -1.7568362275 1.5186695533 1.2826042030 H -0.5592940377 1.7449530375 0.3694007293 $end $rem METHOD b3lyp BASIS cc-pvtz ESP_GRID 0 ESP_EFIELD 2 ! compute E-field on atomic positions $end