lmp: improve error message when compute/fix is not found (#3801)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Bug Fixes**
- Enhanced error handling to ensure required computes or fixes are
present before proceeding with operations, improving robustness and
reliability.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Han Wang <92130845+wanghan-iapcm@users.noreply.github.com>

source/lmp/pair_deepmd.cpp

@@ -219,7 +219,9 @@ void PairDeepMD::make_fparam_from_compute(vector<double> &fparam) {
   int icompute = modify->find_compute(compute_fparam_id);
   Compute *compute = modify->compute[icompute];
 
-  assert(compute);
+  if (!compute) {
+    error->all(FLERR, "compute id is not found: " + compute_fparam_id);
+  }
   fparam.resize(dim_fparam);
 
   if (dim_fparam == 1) {
@@ -246,7 +248,9 @@ void PairDeepMD::make_aparam_from_compute(vector<double> &aparam) {
   int icompute = modify->find_compute(compute_aparam_id);
   Compute *compute = modify->compute[icompute];
 
-  assert(compute);
+  if (!compute) {
+    error->all(FLERR, "compute id is not found: " + compute_aparam_id);
+  }
   int nlocal = atom->nlocal;
   aparam.resize(static_cast<size_t>(dim_aparam) * nlocal);
 
@@ -277,7 +281,9 @@ void PairDeepMD::make_ttm_fparam(vector<double> &fparam) {
       ttm_fix = dynamic_cast<FixTTMDP *>(modify->fix[ii]);
     }
   }
-  assert(ttm_fix);
+  if (!ttm_fix) {
+    error->all(FLERR, "fix ttm id is not found: " + ttm_fix_id);
+  }
 
   fparam.resize(dim_fparam);
 
@@ -316,7 +322,9 @@ void PairDeepMD::make_ttm_aparam(vector<double> &daparam) {
       ttm_fix = dynamic_cast<FixTTMDP *>(modify->fix[ii]);
     }
   }
-  assert(ttm_fix);
+  if (!ttm_fix) {
+    error->all(FLERR, "fix ttm id is not found: " + ttm_fix_id);
+  }
   // modify
   double **x = atom->x;
   int *mask = atom->mask;