diff --git a/qpms/argproc.py b/qpms/argproc.py
index 45a08f9..533470f 100644
--- a/qpms/argproc.py
+++ b/qpms/argproc.py
@@ -193,6 +193,15 @@ def material_spec(string):
                 raise argparse.ArgumentTypeError("Material specification must be a supported material name %s, or a number" % (str(lorentz_drude.keys()),)) from ve
         return lemat
 
+def string2hashable_complex_matrix(string):
+    """
+    Converts string to a hashable equivalent of two-dimensional 
+    numpy.ndarray(..., dtype=complex) (which by itself is not hashable)
+    """
+    matrix = np.ndarray(ast.literal_eval(string), dtype=complex)
+    # TODO here could be some dimensionality checks etc.
+    return tuple(tuple(row) for row in matrix)
+
 def string2bspec(string):
     """
     Converts string representation of list to BaseSpec.
@@ -263,6 +272,12 @@ class ArgParser:
         mpgrp.add_argmuent("+w", "++vswf-set", nargs=2, default={},
                 action=make_dict_action(argtype=string2basespec, postaction='store', first_is_key=True),
                 help='Manual specification of VSWF set codes (format as a python list of integers); see docs on qpms_uvswfi_t for valid codes or simply use ++lMax instead. Overrides ++lMax and --lMax.')
+        mpgrp.add_argmuent("-T", "--constant-tmatrix", nargs=1, default={},
+                action=make_dict_action(argtype=string2hashable_complex_matrix, postaction='store', first_is_key=False),
+                help='constant T-matrix (elements must correspond to --vswf-set)')
+        mpgrp.add_argmuent("+T", "++constant-tmatrix", nargs=2, default={},
+                action=make_dict_action(argtype=string2hashable_complex_matrix, postaction='store', first_is_key=True),
+                help='constant T-matrix (elements must correspond to ++vswf-set)')
 
     atomic_arguments = {
             'rectlattice2d_periods': lambda ap: ap.add_argument("-p", "--period", type=float, nargs='+', required=True, help='square/rectangular lattice periods', metavar=('px','[py]')),