diff --git a/src/libnptm/tfrfme.cpp b/src/libnptm/tfrfme.cpp
index 37b872e209365c6cf71efbab931f6a26ca1ac8db..2b81215780af3610fd3c535a49326a5663780823 100644
--- a/src/libnptm/tfrfme.cpp
+++ b/src/libnptm/tfrfme.cpp
@@ -81,7 +81,7 @@ Swap1* Swap1::from_hdf5(const std::string& file_name) {
   if (status == 0) {
     status = hdf_file->read("NLMMT", "INT32", &_nlmmt);
     status = hdf_file->read("NKV", "INT32", &_nkv);
-    lm = (int)((-2.0 + sqrt(4.0 + 2.0 * _nlmmt)) / 2.0);
+    lm = (int)(sqrt(4.0 + 2.0 * _nlmmt) / 2.0) - 1;
     num_elements = 2 * _nlmmt * _nkv * _nkv;
     instance = new Swap1(lm, _nkv);
     elements = new double[num_elements]();
@@ -107,7 +107,7 @@ Swap1* Swap1::from_legacy(const std::string& file_name) {
   input.open(file_name.c_str(), ios::in | ios::binary);
   if (input.is_open()) {
     input.read(reinterpret_cast<char *>(&_nlmmt), sizeof(int));
-    lm = (int)((-2.0 + sqrt(4.0 + 2.0 * _nlmmt)) / 2.0);
+    lm = (int)(sqrt(4.0 + 2.0 * _nlmmt) / 2.0) - 1;
     input.read(reinterpret_cast<char *>(&_nkv), sizeof(int));
     instance = new Swap1(lm, _nkv);
     int num_elements = _nlmmt * _nkv * _nkv;