Bugs in EXX when nbgrp>1
- On ACE code-path vexx_k (!80 (merged)), vexx_gamma (!91 (merged)) are fixed.
- Non ACE code-path exxenergy2_k and exxenergy2_gamma have remaining issues. (!181 (merged))
- Stress calculation is also buggy with band groups, pw_pbe0 and pw_hse gives NaN when nbgrp=3. fixed in (!93 (merged) and !96 (merged))
Previous investigation shows exxbuff_comm_gamma has invalid memory access which causes segmentation faults. exxbuff_comm is also suspicious. These two routines can be avoided as 1 shows.
Edited by Ye Luo