diff --git a/pyscf/lib/CMakeLists.txt b/pyscf/lib/CMakeLists.txt
index 67bc57acd..77b4cd83a 100644
--- a/pyscf/lib/CMakeLists.txt
+++ b/pyscf/lib/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required (VERSION 3.5)
+cmake_minimum_required (VERSION 3.15)
 project (pyscf_forge)
 
 # Build only the GAS FCI kernel library.  This mode is intended for
@@ -93,11 +93,22 @@ if(ENABLE_OPENMP)
   endif()
 endif()
 
-option(PYSCF_SOURCE_DIR "Pyscf source directory e.g. /usr/local/lib/python3.7/site-packages/pyscf")
+set(PYSCF_SOURCE_DIR "" CACHE PATH "Pyscf source directory e.g. /usr/local/lib/python3.7/site-packages/pyscf")
 if(NOT PYSCF_SOURCE_DIR)
+  # Locate the installed pyscf with the interpreter the build runs under (setup.py passes -DPython_EXECUTABLE),
+  # from the build directory: run from the checkout, an interpreter without pyscf would import the checkout's own
+  # namespace-package portion pyscf/ and answer with the wrong directory, silently.
+  find_package(Python COMPONENTS Interpreter REQUIRED)
   execute_process(
-    COMMAND python3 -c "import pyscf; print(pyscf.__path__[0], end='')"
-    OUTPUT_VARIABLE PYSCF_SOURCE_DIR)
+    COMMAND ${Python_EXECUTABLE} -c "import pyscf; print(pyscf.__path__[0], end='')"
+    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+    OUTPUT_VARIABLE PYSCF_SOURCE_DIR
+    RESULT_VARIABLE _pyscf_rc)
+  if(NOT _pyscf_rc EQUAL 0 OR NOT EXISTS "${PYSCF_SOURCE_DIR}/lib/config.h.in")
+    message(FATAL_ERROR
+      "Could not locate an installed pyscf using ${Python_EXECUTABLE}. "
+      "Pass -DPYSCF_SOURCE_DIR=<.../site-packages/pyscf> or -DPython_EXECUTABLE=<interpreter with pyscf>.")
+  endif()
 endif()
 message(STATUS "Include pyscf source dir: ${PYSCF_SOURCE_DIR}")
 include_directories(${PYSCF_SOURCE_DIR}/lib ${PYSCF_SOURCE_DIR}/lib/deps/include)
diff --git a/setup.py b/setup.py
index 7cd3a4a19..c4ca84a3f 100755
--- a/setup.py
+++ b/setup.py
@@ -98,6 +98,9 @@ def run(self):
         self.announce('Configuring extensions', level=3)
         src_dir = os.path.abspath(os.path.join(__file__, '..', 'pyscf', 'lib'))
         cmd = ['cmake', f'-S{src_dir}', f'-B{self.build_temp}']
+        # The interpreter this build runs under: CMakeLists.txt asks it where pyscf is installed, so that the lookup
+        # does not depend on which python3 is first on PATH.
+        cmd.append(f'-DPython_EXECUTABLE={sys.executable}')
         
         # Allow user to disable OCCRI C extension build
         build_occri = os.getenv('BUILD_OCCRI', 'ON')