diff --git a/tests/test_opt_requirements.txt b/tests/test_opt_requirements.txt
index cdd418a..53d169e 100644
--- a/tests/test_opt_requirements.txt
+++ b/tests/test_opt_requirements.txt
@@ -1,4 +1,5 @@
 mkl>=2025.0.1
 mkl-include
+pybind11
 block2 --extra-index-url=https://block-hczhai.github.io/block2-preview/pypi/
 git+https://github.com/pyscf/dmrgscf