Slides

QM/MM: Electrostatic interaction
• Plane waves: Simple numerical evalua4on of the coupling term is
prohibi4ve as it would involve a large number of opera4ons:
• Electronic charge density n(r) is defined on a FFT grid in both real
and reciprocal space:
Number of grid points (QM) × Number of MM atoms
E es
QM −MM
n(r)
= ∑ q I ∫
i∈MM r − R I
• Electron spill out or charge leakage due to the lack of orthogonality
and thus Pauli repulsion for the interac4on of the electrons in the QM
part with the nearby MM atoms.
• Par4cularly severe for plane-­‐wave basis sets
• The nega4ve electronic density can easily spread out into the
vacuum region, where the posi4ve par4al charges of the MM
atoms are located, instead of being kept close to the QM nuclei as
is the case if small Gaussian basis sets without diffuse func4ons
are used.
• Various coupling schemes devised to cope with this problem
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