[BUG] Two issues with pair_style extep

[mkanski]

Summary

I found two issues with pair_style extep:

1. It requires that all elements from the potential file are used in pair_coeff.
2. It does not work when NULL is used in pair_coeff.

LAMMPS Version and Platform

Current master, Linux

Expected Behavior

1. It should be possible to use line like pair_coeff B.
2. When using extep with pair_style hybrid line like pair_coeff * * BN.extep B N NULL should be accepted.

Actual Behavior

1. LAMMPS quits with an error: "ERROR: Incorrect format in ExTeP potential file (../pair_extep.cpp:752)"
2. LAMMPS quits with an error: "ERROR: Insufficient spline parameters in potential file (../pair_extep.cpp:644)"

Steps to Reproduce

First:
region u sphere 0 0 0 10
create_box 1 u
pair_style extep
pair_coeff * * BN.extep N

Second:
region u sphere 0 0 0 10
create_box 3 u
pair_style hybrid/overlay extep lj/cut 4.0
pair_coeff * * extep BN.extep N B NULL

Further Information, Files, and Links

I'm not sure what is the cause for the first problem, but the second issue can be resolved by changing atom->ntypes to nelements in line:

lammps/src/USER-MISC/pair_extep.cpp

Line 708 in 0849e03

if (nparams >= pow(atom->ntypes,3)) break;

EDIT: The change fixes problem only if NULL is the last atom type(s), so it isn't a whole solution.

[akohlmey]

Am looking into it. There are several places where edits are needed for systems with only one element or for use with pair style hybrid.

[mkanski]

I think I solved it, please see the PR.