throw runtime error when SVD on no block tensors

Author: QuantumLiquids

include/qlten/mpi_tensor_manipulation/ten_decomp/mpi_svd.h

@@ -75,6 +75,16 @@ void MPITensorSVDExecutor<TenElemT, QNT>::Execute(void) {
       mpi_comm_
   );
   auto kept_sv_info = this->CalcTruncedSVInfo_(idx_raw_data_svd_res);
+
+  // Empty SVD result: input tensor has no valid blocks for this QN configuration
+  if (kept_sv_info.empty()) {
+    DeleteDataBlkMatSvdResMap(idx_raw_data_svd_res);
+    throw std::runtime_error(
+        "MPI SVD failed: empty result. The input tensor has no data blocks "
+        "compatible with the specified quantum number divergence (lqndiv)."
+    );
+  }
+
   this->ConstructSVDResTens_(kept_sv_info, idx_raw_data_svd_res);
   DeleteDataBlkMatSvdResMap(idx_raw_data_svd_res);
 

include/qlten/tensor_manipulation/ten_decomp/ten_svd.h

@@ -13,8 +13,9 @@
 #ifndef QLTEN_TENSOR_MANIPULATION_TEN_DECOMP_TEN_SVD_H
 #define QLTEN_TENSOR_MANIPULATION_TEN_DECOMP_TEN_SVD_H
 
-#include <cassert>     // assert
-#include <utility>    // pair
+#include <cassert>      // assert
+#include <utility>      // pair
+#include <stdexcept>    // runtime_error
 
 #include "qlten/framework/bases/executor.h"                           // Executor
 #include "qlten/qltensor_all.h"
@@ -192,6 +193,16 @@ void TensorSVDExecutor<TenElemT, QNT>::Execute(void) {
       idx_ten_decomp_data_blk_mat_map_
   );
   auto kept_sv_info = CalcTruncedSVInfo_(idx_raw_data_svd_res);
+
+  // Empty SVD result: input tensor has no valid blocks for this QN configuration
+  if (kept_sv_info.empty()) {
+    DeleteDataBlkMatSvdResMap(idx_raw_data_svd_res);
+    throw std::runtime_error(
+        "SVD failed: empty result. The input tensor has no data blocks "
+        "compatible with the specified quantum number divergence (lqndiv)."
+    );
+  }
+
   ConstructSVDResTens_(kept_sv_info, idx_raw_data_svd_res);
   DeleteDataBlkMatSvdResMap(idx_raw_data_svd_res);
 

tests/test_tensor_manipulation/test_ten_svd.cc

@@ -697,6 +697,23 @@ TEST_F(TestSvd, 3DCaseFloat) {
       &qn0);
 }
 
+// Test that SVD throws when input tensor has no data blocks
+TEST_F(TestSvd, EmptySVDResultThrows) {
+  // Create a tensor without any data blocks (not initialized with Random)
+  DQLTensor empty_tensor({idx_in_s, idx_out_s});
+  // Don't call Random() - tensor has structure but no actual data blocks
+
+  DQLTensor u, vt;
+  QLTensor<QLTEN_Double, U1QN> s;
+  QLTEN_Double trunc_err;
+  size_t D;
+
+  EXPECT_THROW(
+      SVD(&empty_tensor, 1, qn0, 0.0, 1, 100, &u, &s, &vt, &trunc_err, &D),
+      std::runtime_error
+  );
+}
+
 struct TestSvdOmpParallel : public testing::Test {
   std::string qn_nm = "qn";
 };