## How to use an Add-On’s name in directory structure, build, and distribution¶

• Select a name. May be mixed case with numerals and underscores (e.g., CheMPS2, libefp, PCMSolver, v2rdm_casscf). Shouldn’t start with a numeral. Needn’t start with “lib”, even if a library.
• GitHub repository name should be AddOn_name or AddOn_name.lower() (hereafter, addon_name. For example: CheMPS2, libefp, pcmsolver, v2rdm_casscf.
• CMake project name should be AddOn_name. For example: project(libefp), project(CheMPS2), project(PCMSolver), project(v2rdm_casscf). Namespacing in the directory structure used to detect the addon should have this name (e.g., share/cmake/CheMPS2).
• Restricted by the CMake project name, add-ons return CMake variables and compile definitions of FOUND_AddOn_name and USING_AddOn_name. For example: FOUND_libefp, USING_CheMPS2, PCMSolver_LIBRARIES, USING_v2rdm_casscf.
• The CMake target(s) formed use the full add-on name as the namespace, AddOn_name::lib_name_without_lib.lower(). For example: libefp::efp, CheMPS2::chemps2, PCMSolver::pcm, v2rdm_casscf::v2rdm_casscf.
• Following the CMake project name (though not restricted to it – PSI4 managment could change the pattern), the user flag to enable an add-on is ENABLE_AddOn_name. Note that runtime-only add-ons don’t go through this enabling process.
• Internally, the ExternalProject_Add and dummy libraries as well as any tests/ and external/ subdirectories should all be lowercase, addon_name.
• The conda package and internal to PSI4 (that is, the ExternalProject_Add, dummy libraries, and any tests/ and external/ subdirectories) should all be lowercase, addon_name.
• Alternatively, you can do everything mentioned here lowercase and just have a different capitalization for an advertising name. After all, that’s what PSI4 does.

## How to integrate an Add-On into build, testing, and docs¶

• In all cases, put Add-Ons in alphabetic order, ignoring any “lib” in the name.
• psi4/CMakeLists.txt
• Add the ENABLE_AddOn_name line
• Add the external_addon_name dependency to the psi4-core external project
• Add the AddOn_name_DIR variable passing to the psi4-core external project
• psi4/psi4/CMakeLists.txt
• Add a block imitating Libint if Add-On required or CheMPS2 if not required
• If there are shared resources to the external that need to be found by PSI4 in PSIDATADIR, follow the efpfrag pattern of libefp to symlink them in.
• psi4/psi4/src/CMakeLists.txt
• No changes should be required unless both (1) code in export_* or core.cc needs the USING_AddOn_name definition or AddOn header includes and (2) no binary PSI4 module (as opposed to library PSI4 module with the AddOn target linked is itself a direct dependency of target core. Basically, try to leave this file alone, but if there are compile errors, add the definitions/headers as needed.
• psi4/psi4/src/psi4/
• If a module is needed to interface the AddOn to PSI4, try to put “interface” in the name. Follow the pattern of CheMPS2 or gdma. If non-required, be sure to conditionalize it with if(TARGET AddOn::addon) in CMake files or #ifdef USING_AddOn in source files.
• If a separate module is not required, follow the patter of dkh or simint with respect to libmints. Again, conditionalize as in preceding bullet.
• psi4/psi4/external/upstream/
• Add a CMakeLists.txt that imitates another AddOn of similar language and dependencies. Try to keep the format, messaging, and variables passed as similar as possible so that differences mean something. If BLAS/LAPACK or other common dependencies in psi4/psi4/common/ are needed, be sure to add them to the DEPENDS argument.
• The usual practice to to get everything cohesive between the CMake for the AddOn repository and PSI4 and then as a last step, mint a tag in the former and add it to two places in psi4/external/upstream/addon_name/CMakeLists.txt and one place in psi4/psi4/CMakeLists.txt so that only that version and later are acceptable to PSI4 for detecting pre-built.
• psi4/tests/
• Create new subdirectory tests/addon_name with a CMakeLists.txt. In that add a few tests. Imitate the pattern in other subdirs of including the addon prefix to the test name in the CMakeLists but not in the test dir name. Make sure the tests get the addon CTest label and that at least one of them gets the smoke label.
• psi4/doc/sphinxman/

## else¶

• Build conda packages
• PSI4 and Add-On Projects Working Together
• Obligations of the External Project owners are to:
1. allow us to contribute some CMake files to your build system so that compile flags and dependencies (e.g., BLAS/LAPACK) can be consistent with the PSI4 build and so the installed project can be readily detected by PSI4 or any interested party (through a CMake imported target).
2. provide us a tag at a tested commit/version number so their development may be ongoing.
3. communicate with us when they’ve made improvements and minted a new tag.
• In return, for Add-Ons the PSI4 project will:
1. leave control of their code under your purview.
2. maintain any interfacing code needed.
3. regularly run integration tests between PSI4 and your code.
4. build a mostly statically linked conda package so that any of your users can obtain a pre-built binary distribution through conda install addon --channel psi4.
5. provide a development sandbox for your code through PSI4 plugins.