ERFC_SPLINE

NAME
ERFC_SPLINE - Use spline to compute the complementary error function used for electrostatics in direct space

SYNOPSIS
ERFC_SPLINE  erfc_bin
ERFC_SPLINE  erfc_bin   corrected rcut

DESCRIPTION
By default ORAC uses a 5 parameter expansion to compute the complementary error function required by the direct space electrostatic potential ($V_{qd}$ in Eq. 4.20). With the command ERFC_SPLINE this expansion is replaced by a B-spline. The function $erfc \left ( x \right )$ is splined from $x=0$ to $x=1.1 \alpha r_{cut}$, where $\alpha$ and $r_{cut}$ are the Ewald sum parameter and the radial cutoff, respectively. The argument erfc_bin is the bin size of the spline. The usage of the ERFC_SPLINE option is useful when running on workstations where a saving of 10-15 % in CPU time is usually obtained. ERFC_SPLINE may also be used to speed up the Ewald method. By specifying the directive corrected ORAC corrects for the reciprocal lattice cutoff for all intermolecular interactions in the direct lattice using the same oscillating potential of Eq. (4.47) (see Sec. 4.4) used for correcting the intra-molecular potential (see ERF_CORR in this environment. This allows the use shorter cutoffs in reciprocal space (or coarser grids in SPME). The argument rcut corresponds to the maximum distance for the spline table. Must be larger than the current cutoff (see examples).

EXAMPLES
ERFC_SPLINE 0.01
A B-spline is used to evaluate the direct space sum. To evaluate the B-spline the original function is computed on a grid of 0.01 bin size.
ERFC_SPLINE 0.01 corrected 14.0
The splined potential is now given by standard the direct lattice Ewald term plus the $\chi(r,\alpha)$ potential defined in Eq. (see also command ERF_CORR in this environment). The B-spline look up table is done for distances $0 < r < 14$.

WARNINGS
rcut is an atomic cutoff. Always define rcut large enough to assure that all atoms are included within rcut for any molecular pair. E. g., if $r_{h}$ the largest cutoff defined in the structured command MTS_RESPA (&INTEGRATOR) and the molecule has a maximum extension in any possible direction of $\Delta R$, choose ${\it rcut} = r_{h} + \Delta R$