The Ewald interaction is a periodic function which differs from
in such a way that the sum of interactions between pairs of particles
*within one cell* gives the exact Coulomb energy per cell of a
periodic lattice of identical cells. This ensures that the Ewald
Hamiltonian gives the correct Hartree energy, but the deviations from
give rise to a spurious contribution to the XC energy,
corresponding to the periodic repetition of the XC
hole. [86,87] A solution is to
change the
many-body Hamiltonian so that the interaction with the XC hole is
exactly (see section 6.7) for any size and shape of
simulation cell, without altering the form of the Hartree energy.
This source of finite size error is therefore eliminated and
equation 6.3, with the correction term calculated within a
the LDA, gives a much better description of the remaining finite size
errors. Greater accuracy can be obtained by adding an extrapolation
term, but this term is much smaller than if the Ewald interaction is
used.

The utility of changing the Hamiltonian to reduce the finite size errors is readily seen by considering an example. Suppose that a periodic boundary condition QMC technique is used to study an isolated molecule. If the molecule is placed at the center of the periodically repeated simulation cell, the calculated energy is that of an array of identical molecules, including unwanted inter-molecular interactions. The results improve as the simulation cell is made larger, but the convergence is slow, especially for molecules with permanent dipole moments. A better solution is to cut off all Coulomb interactions between charges on different molecules, i.e., to replace the Ewald interactions by truncated Coulomb interactions acting only within the simulation cell. As long as the molecular wave function has decayed to zero before the simulation cell boundary is reached, this procedure should give essentially exact results.

An alternative procedure is to evaluate the correction term in equation 6.3 using HF data. HF theory is an approximate method for solving the many-body Hamiltonian, and if the Ewald formula is used for the electron-electron interaction terms in both the many-body and HF theories, the finite size error in the HF exchange energy will tend to cancel the finite size error in the many-body interaction energy. However, this procedure gives too large a correction, presumably because the HF exchange hole is significantly different from the screened XC hole of the many-body system.