Dear Hakim,
Please follow the code path in subroutine usrshl in dyn21ushl.F for nipp.eq.0 - this path calls usrshl_h, which in turn calls ushl_e101 etc for computing the force and stiffness. Subroutines ushl_e101 and ushl_e104 have sample code.
If there is still and issue, the best approach for debugging is to print out various quantities and see what may be going wrong.
Cheers
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