Approximation in tail correction better explained

NOTE: The analytical form of the tail corrections for energy and pressure used

in the {lj/cut/soft} potentials are approximate, being identical to that of the

corresponding non-soft potentials scaled by a factor lambda^n. For real-space

cutoffs greater than 2.5 sigma and alpha_LJ = 0.5, the error affecting the tail

correction is below 0.2% when lambda = 0 (worse case). This error disappears as

lambda approaches 1. Note that this is the error affecting the long-range tail

(itself a correction to the interaction energy) which includes other

approximations, namely that the system is homogenous (local density equal the

average density) beyond the cutoff.

corresponding non-soft potentials scaled by a factor lambda^n. The errors due to

this approximation should be negligible. For example, for a cutoff of 2.5 sigma

this approximation leads to maximum relative errors in tail corrections of the

order of 1e-4 for energy and virial (alpha_LJ = 0.5, n = 2). The error vanishes

when lambda approaches 0 or 1. Note that these are the errors affecting the

long-range tail (itself a correction to the interaction energy) which includes

other approximations, namely that the system is homogenous (local density equal

the average density) beyond the cutoff.

The {morse/soft} pair style does not support the "pair_modify"_pair_modify.html

tail option for adding long-range tail corrections to energy and pressure.