Addition of an electric field without point charges

Questions
#1

Hi everyone,

I am attempting to run some 2D PES of a molecule in an electric field and I was wondering if there were some sort of method to add an electric field at a specified orientation without the use of point charges. Thank you!

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#2

There is, by adding a $multipole_field section to your input file. This is described (briefly!) in Appendix C of the Q-Chem manual. In a little more detail: an electric field looks like a term proportional to x, y, and/or z in your Hamiltonian:
H = H(molecule) + H(field) = H(molecule) + Ex \Hat{x} + Ey \Hat{y} + Ez \Hat{z}
where the E-field vector is E = (Ex,Ey,Ez). Following the format stipulated in App. C, the appropriate thing to add to your input section is therefore

$multipole_field
X Ex
Y Ey
Z Ez
$end

where Ex, Ey, and Ez are real numbers, giving the field strength in atomic units (Hartree/bohr).