From d4947525e4c90289735801c26c4e5ca4a8d8aa33 Mon Sep 17 00:00:00 2001
From: Miles Stoudenmire <miles.stoudenmire@gmail.com>
Date: Mon, 8 Feb 2021 12:04:53 -0500
Subject: [PATCH] Add error checks for zero blocks in randomITensor QN version

---
 itensor/itensor.cc | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/itensor/itensor.cc b/itensor/itensor.cc
index fd4240c32..175529796 100644
--- a/itensor/itensor.cc
+++ b/itensor/itensor.cc
@@ -1580,6 +1580,7 @@ randomITensor(QN q, IndexSet const& is)
     ITensor T;
     auto dat = QDenseReal{is,q};
     T = ITensor(std::move(is),std::move(dat));
+    if(nnz(T) == 0) Error("Requested QN for random ITensor resulted in zero allowed blocks (QN not satisfiable by any settings of the indices)");
     T.generate(detail::quickran);
     return T;
     }
@@ -1594,6 +1595,7 @@ randomITensorC(QN q, IndexSet const& is)
     ITensor T;
     auto dat = QDenseCplx{is,q};
     T = ITensor(std::move(is),std::move(dat));
+    if(nnz(T) == 0) Error("Requested QN for random ITensor resulted in zero allowed blocks (QN not satisfiable by any settings of the indices)");
     T.generate(detail::quickranCplx);
     return T;
     }