Wrong dispersion correction for Mg

[KaiSellschopp]

Describe the bug
I believe there is an error in the calculation of the dispersion correction for Mg. When I calculate the dispersion energy for different lattice constants, the dispersion correction "jumps" to less negative values for smaller lattice constants.

To Reproduce
Steps to reproduce the behaviour:

1. happens with input (include input files)
   • Take the hcp structure of Mg (e.g. from COD or from attached file)
   • Change the lattice vectors by a factor (e.g. in the range 0.95 to 1.05)
2. start dftd4 with (all the options here) dftd4 --func pbe POSCAR (but same behaviour occurs for other functionals, I also tried pbesol and scan)
3. output showing the error: there is a discontinuous jump in the dispersion energy (see attached files)

I attached the Mg structure file from the COD (and converted to POSCAR format), and a small script that changes the lattice constant and starts dftd4 to illustrate the problem. You can start it like this bash d4_lattice_constant.sh [STRUCTURE] [FUNCTIONAL], where [STRUCTURE] is the name of the structure file (POSCAR format) and [FUNCTIONAL] is the option passed to --func. The attached "*.summary" files are some example outputs that I get, and I also plotted them to illustrate the problem.

Expected behaviour
I would expect a continuous increase in the dispersion energy (i.e. to less negative values) with increasing lattice constant. This is also what I observe for other materials. I tested Al and Li, structure files for both are attached for comparison, for Li I also attached the output of my script.

Additional context
I use the latest release of DFT-D4 and compiled it from the source code. The compilation and tests ran without any errors. There was only a warning that the option "-fortranlibs" is not known to the compiler and ignored:

[253/290] Linking Fortran executable _deps/multicharge-build/app/multicharge
ifort: command line warning #10006: ignoring unknown option '-fortranlibs'
[278/290] Linking Fortran executable _deps/multicharge-build/test/multicharge-tester
ifort: command line warning #10006: ignoring unknown option '-fortranlibs'
[286/290] Linking Fortran executable app/dftd4
ifort: command line warning #10006: ignoring unknown option '-fortranlibs'
[290/290] Linking Fortran executable test/unit/dftd4-tester
ifort: command line warning #10006: ignoring unknown option '-fortranlibs

dftd4_issue_Mg.zip
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