eOn Potentials Over rgpot

The **eOn ↔ rgpot** integration gives eOn access to rgpot’s quantum-chemistry backends (**NWChem** via nwchemc, **CPMD** e.g. BLYP via cpmdc) through two independent roles:

  • **In-process (shipped in eOn):** eOn’s RgpotPot (-Dwith_rgpot=true) links

rgpot’s NWChemPot / CPMDPot frontends as a Meson subproject and dlopen=s the engine libraries directly. No sockets, no potserv. Configured through the eOn =[RgpotPot] INI section (backend, basis, theory, scf_type, functional, charge, multiplicity, engine_library, …).

  • **Out-of-process RPC (this repo):** potserv <port> NWChem|CPMD serves the

Cap’n Proto Potential interface for any client; eOn can also act as an RPC server via eonclient --serve (-Dwith_serve=true).

Engine builds live in nwchemc / cpmdc; rgpot frontends dlopen them at runtime in both roles.

Why This Shape

  • configure(PotentialConfig) once per method (NWChemParams / CPMDParams).

  • calculate(ForceInput) per geometry (positions, atomic numbers, cell, units).

  • Units on the wire (lengthUnit / energyUnit); eOn’s RgpotPot converts

Hartree → eV and Hartree/bohr → eV/Å at the boundary.

  • Engines are optional at build time: CI uses fake libnwchemc / libcpmdc;

production sets NWCHEMC_LIBRARY / CPMDC_LIBRARY (or aliases).

Landed Work Items

  1. **eOn potential class** (eOn tree): client/potentials/Rgpot/ —

RgpotPot (eOn Potential subclass) over an opaque RGPotEngine TU that isolates rgpot / Cap’n Proto headers from eOn’s own Potential type.

  1. **CPMD BLYP example** (eOn tree): examples/rgpot_cpmd_blyp/ with the

in-process [RgpotPot] configuration.

  1. **NWChem comparison** (eOn tree): scripts/bench_rgpot_vs_socket_forces.sh

and scripts/rgpot_nwchem_vs_socket.sh compare RgpotPot against eOn’s SocketNWChem on a fixed deck class.

  1. **rgpot ergonomics** (this repo): potentials-schema v1.5.0 parity (typed

NWChem/CPMD arms, metatomic outputs surface), schema-sync CI gate, and the in- process CPMD timing harness (scripts/cpmd_inprocess_vs_subprocess_timing.sh).

Operator Cheatsheet (rgpot side)

# Real engines (built in OmniPotentRPC/nwchemc and OmniPotentRPC/cpmdc)
NWCHEMC_LIBRARY=/path/to/libnwchemc.so ./bbdir/CppCore/potserv 12345 NWChem
CPMDC_LIBRARY=/path/to/libcpmdc.so   ./bbdir/CppCore/potserv 12346 CPMD

# Fake engines (meson -Dwith_rpc=true -Dwith_tests=true)
export RGPOT_POTSERV=./bbdir/CppCore/potserv
export NWCHEMC_LIBRARY=./bbdir/CppCore/libnwchemc_fake_engine.so
export RGPOT_CPMD_ENGINE=./bbdir/CppCore/libcpmdc_fake_engine.so
pixi run -e rpctest -- python tests/test_rpc_e2e_c_abi.py

See CppCore/rgpot/NWChemPot/README.md, CppCore/rgpot/CPMDPot/README.md, and docs/orgmode/howto/integration.org.