Found during a Codex global scan of deepmodeling/deepmd-kit at commit 73de44b1f94471b2e3bdb6b11f57b34d7bc791bb.

Problem

Magnetic cell-derivative output definitions are inserted into the coordinate-derivative bucket instead of the cell-derivative bucket.

Evidence:

• ModelOutputDef stores coordinate derivatives in def_derv_r, cell derivatives in def_derv_c, and builds reduced cell derivatives from def_derv_c:

  deepmd-kit/deepmd/dpmodel/output_def.py

  Lines 294 to 310 in 73de44b

  	self.def_outp = fit_defs
  	self.def_redu = do_reduce(self.def_outp.get_data())
  	self.def_derv_r, self.def_derv_c = do_derivative(self.def_outp.get_data())
  	self.def_hess_r, _ = do_derivative(self.def_derv_r)
  	self.def_derv_c_redu = do_reduce(self.def_derv_c)
  	self.def_mask = do_mask(self.def_outp.get_data())
  	self.var_defs: dict[str, OutputVariableDef] = {}
  	for ii in [
  	self.def_outp.get_data(),
  	self.def_redu,
  	self.def_derv_c,
  	self.def_derv_r,
  	self.def_derv_c_redu,
  	self.def_hess_r,
  	self.def_mask,
  	]:
  	self.var_defs.update(ii)

• keys_derv_c() returns self.def_derv_c.keys():

  deepmd-kit/deepmd/dpmodel/output_def.py

  Lines 338 to 342 in 73de44b

  	def keys_derv_c(self): # noqa: ANN201
  	return self.def_derv_c.keys()
  	def keys_derv_c_redu(self): # noqa: ANN201
  	return self.def_derv_c_redu.keys()

• In do_derivative(), the non-magnetic cell derivative is correctly placed in def_derv_c:

  deepmd-kit/deepmd/dpmodel/output_def.py

  Lines 503 to 513 in 73de44b

  	if vv.c_differentiable:
  	assert vv.r_differentiable
  	def_derv_c[rkc] = OutputVariableDef(
  	rkc,
  	vv.shape + [9], # noqa: RUF005
  	reducible=True,
  	r_differentiable=False,
  	c_differentiable=False,
  	atomic=True,
  	category=apply_operation(vv, OutputVariableOperation.DERV_C),
  	)

• The magnetic cell derivative rkcm is instead assigned into def_derv_r:

  deepmd-kit/deepmd/dpmodel/output_def.py

  Lines 514 to 524 in 73de44b

  	if vv.magnetic:
  	def_derv_r[rkcm] = OutputVariableDef(
  	rkcm,
  	vv.shape + [9], # noqa: RUF005
  	reducible=True,
  	r_differentiable=False,
  	c_differentiable=False,
  	atomic=True,
  	category=apply_operation(vv, OutputVariableOperation.DERV_C),
  	magnetic=True,
  	)

Impact

keys_derv_c() misses magnetic cell derivatives, and do_reduce(self.def_derv_c) cannot create the corresponding reduced magnetic virial definition. Magnetic models can therefore omit or misclassify *_derv_c_mag outputs even when the fitting output is cell-differentiable.

Suggested Fix

Change the magnetic cell-derivative assignment from def_derv_r[rkcm] to def_derv_c[rkcm]. Add a unit test with a magnetic, cell-differentiable output definition that asserts *_derv_c_mag appears in keys_derv_c() and its reduced definition is present.