diff --git a/external/class.cmake b/external/class.cmake
index ab2e691..0a3f3c2 100644
--- a/external/class.cmake
+++ b/external/class.cmake
@@ -1,75 +1,139 @@
-# initialize the class submodule if necessary
-if(EXISTS "${CMAKE_CURRENT_LIST_DIR}/class/Makefile")
-  message(STATUS "class submodule is initialized.")
-else()
-  message(STATUS "class submodule is NOT initialized: executing git command")
-  execute_process(COMMAND git submodule update --init -- external/class
-    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
-endif()
+option(ENABLE_CLASS "Enable CLASS support (as a submodule)." ON)
 
-set(CLASS_INCLUDE_DIR ${CMAKE_CURRENT_LIST_DIR}/class/include)
-set(CLASS_INCLUDE_CPP_DIR ${CMAKE_CURRENT_LIST_DIR}/class/cpp)
+if(ENABLE_CLASS)
 
-# list of object files generated by class
-set(CLASS_OBJECT_FILES
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/arrays.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/background.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/common.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/dei_rkck.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/evolver_ndf15.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/evolver_rkck.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/growTable.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/helium.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/history.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/hydrogen.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/hyperspherical.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/hyrectools.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/input.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/lensing.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/nonlinear.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/output.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/parser.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/perturbations.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/primordial.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/quadrature.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/sparse.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/spectra.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/thermodynamics.o
-  ${CMAKE_CURRENT_LIST_DIR}/class/build/transfer.o
-)
+  # initialize the class submodule if necessary
+  if(EXISTS "${CMAKE_CURRENT_LIST_DIR}/class/Makefile")
+    message(STATUS "class submodule is initialized.")
+  else()
+    message(STATUS "class submodule is NOT initialized: executing git command")
+    execute_process(COMMAND git submodule update --init -- external/class
+      WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
+  endif()
 
-# python3
-find_package(Python3 REQUIRED COMPONENTS Interpreter)
+  # include directories
+  set(CLASS_INCLUDE_DIR ${CMAKE_CURRENT_LIST_DIR}/class/include)
+  set(CLASS_INCLUDE_CPP_DIR ${CMAKE_CURRENT_LIST_DIR}/class/cpp)
 
-# command to build class using its own makefile
-add_custom_command(OUTPUT ${CLASS_OBJECT_FILES}
-  COMMAND PYTHON=${Python3_EXECUTABLE} CC=${CMAKE_C_COMPILER} make
-  WORKING_DIRECTORY ${CMAKE_CURRENT_LIST_DIR}/class
-)
+  # build method for class
+  option(USE_CLASS_MAKEFILE "Build CLASS using its own Makefile." OFF)
 
-# target for class objects
-add_custom_target(class_objects DEPENDS ${CLASS_OBJECT_FILES})
+  if(USE_CLASS_MAKEFILE)
+    # list of object files generated by class
+    set(CLASS_OBJECT_FILES
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/arrays.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/background.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/common.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/dei_rkck.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/evolver_ndf15.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/evolver_rkck.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/growTable.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/helium.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/history.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/hydrogen.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/hyperspherical.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/hyrectools.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/input.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/lensing.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/nonlinear.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/output.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/parser.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/perturbations.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/primordial.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/quadrature.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/sparse.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/spectra.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/thermodynamics.o
+      ${CMAKE_CURRENT_LIST_DIR}/class/build/transfer.o
+    )
 
-# library for class cpp wrappers
-add_library(class_cpp 
-  ${CMAKE_CURRENT_LIST_DIR}/class/cpp/Engine.cc
-  ${CMAKE_CURRENT_LIST_DIR}/class/cpp/ClassEngine.cc)
-target_include_directories(class_cpp
-  PRIVATE ${CMAKE_CURRENT_LIST_DIR}/class/include)
+    # python3
+    find_package(Python3 REQUIRED COMPONENTS Interpreter)
+
+    # command to build class using its own makefile
+    add_custom_command(OUTPUT ${CLASS_OBJECT_FILES}
+      COMMAND PYTHON=${Python3_EXECUTABLE} CC=${CMAKE_C_COMPILER} make
+      WORKING_DIRECTORY ${CMAKE_CURRENT_LIST_DIR}/class
+    )
+
+    # target for class objects
+    add_custom_target(class_objects DEPENDS ${CLASS_OBJECT_FILES})
+
+    # library for class cpp wrappers
+    add_library(class_cpp 
+      ${CMAKE_CURRENT_LIST_DIR}/class/cpp/Engine.cc
+      ${CMAKE_CURRENT_LIST_DIR}/class/cpp/ClassEngine.cc)
+    target_include_directories(class_cpp
+      PRIVATE ${CMAKE_CURRENT_LIST_DIR}/class/include)
+  else(USE_CLASS_MAKEFILE)
+
+    # list of CLASS source files
+    set(CLASS_SOURCE_FILES
+      ${CMAKE_CURRENT_LIST_DIR}/class/tools/growTable.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/tools/dei_rkck.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/tools/sparse.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/tools/evolver_rkck.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/tools/evolver_ndf15.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/tools/arrays.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/tools/parser.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/tools/quadrature.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/tools/hyperspherical.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/tools/common.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/source/input.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/source/background.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/source/thermodynamics.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/source/perturbations.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/source/primordial.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/source/nonlinear.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/source/transfer.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/source/spectra.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/source/lensing.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/hyrec/hyrectools.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/hyrec/helium.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/hyrec/hydrogen.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/hyrec/history.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/source/output.c
+      ${CMAKE_CURRENT_LIST_DIR}/class/cpp/Engine.cc
+      ${CMAKE_CURRENT_LIST_DIR}/class/cpp/ClassEngine.cc
+    )
+    
+    # create the library
+    add_library(class ${CLASS_SOURCE_FILES})
+    target_include_directories(class PRIVATE ${CLASS_INCLUDE_DIR})
+    target_include_directories(class PRIVATE ${CLASS_INCLUDE_CPP_DIR})
+    target_compile_options(class PRIVATE "-ffast-math")
+    target_compile_options(class PRIVATE "-D__CLASSDIR__=\"${CMAKE_CURRENT_LIST_DIR}/class\"")
+    set_property(TARGET class PROPERTY POSITION_INDEPENDENT_CODE ON)
+    target_include_directories(class PRIVATE ${CMAKE_CURRENT_LIST_DIR}/class/hyrec)
+    target_compile_options(class PRIVATE "-DHYREC")
+    set_target_properties(class PROPERTIES CXX_STANDARD 14 C_STANDARD 11)
+    # target_compile_options(class PRIVATE "-Wall")
+    # target_compile_options(class PRIVATE "-Wextra")
+    # target_compile_options(class PRIVATE "-pedantic")
+  endif(USE_CLASS_MAKEFILE)
+endif(ENABLE_CLASS)
 
 # macro to setup include dir and link libraries for target using class
 macro(target_setup_class target_name)
-  target_include_directories(${target_name}
-    PRIVATE ${CLASS_INCLUDE_DIR})
-  target_include_directories(${target_name}
-    PRIVATE ${CLASS_INCLUDE_CPP_DIR})
-  target_link_libraries(${target_name} ${CLASS_OBJECT_FILES})
-  target_link_libraries(${target_name} class_cpp)
-  add_dependencies(${target_name} class_objects)
+  if(ENABLE_CLASS)
+    target_include_directories(${target_name}
+      PRIVATE ${CLASS_INCLUDE_DIR})
+    target_include_directories(${target_name}
+      PRIVATE ${CLASS_INCLUDE_CPP_DIR})
+    if(USE_CLASS_MAKEFILE)
+      target_link_libraries(${target_name} ${CLASS_OBJECT_FILES})
+      target_link_libraries(${target_name} class_cpp)
+      add_dependencies(${target_name} class_objects)
+    else(USE_CLASS_MAKEFILE)
+      target_link_libraries(${target_name} class)
+    endif(USE_CLASS_MAKEFILE)
+    target_compile_options(${target_name} PRIVATE "-DUSE_CLASS")
+  endif(ENABLE_CLASS)
 endmacro(target_setup_class)
 
-
-# test executable
-add_executable(testTk
-  ${CMAKE_CURRENT_LIST_DIR}/class/cpp/testTk.cc)
-target_setup_class(testTk)
\ No newline at end of file
+if(ENABLE_CLASS)
+  # test executable
+  add_executable(testTk
+    ${CMAKE_CURRENT_LIST_DIR}/class/cpp/testTk.cc)
+  target_setup_class(testTk)
+endif(ENABLE_CLASS)
\ No newline at end of file