PyMES

Python package for many-electron simulations — coupled-cluster theory for ground states, excited states, and real-time dynamics, with first-class support for non-Hermitian transcorrelated Hamiltonians

Repo: nickirk/pymes

What it is

PyMES is a Python package for many-electron simulations that I have developed and maintained as open source since my PhD years. It implements coupled-cluster theory for both ground and excited states, with first-class support for non-Hermitian Hamiltonians — the capability that makes transcorrelated and similarity-transformed methods tractable. It is the reference codebase in which I have developed and validated several published methods, and it continues to be the platform for my work on excited states and real-time electron dynamics.

What’s inside

  • Ground-state coupled cluster — MP2, CCD, DCD, CCSD, DCSD.
  • Excited states — EOM-CCSD, FEAST-EOM-CCSD (energy-window targeting), and my own contour-integral / Cauchy-integral-formula (CIF) variant that combines black-box eigenvalue targeting with real-time propagation.
  • Real-time electron dynamics — RT-EOM-CCSD built on the CIF representation of the time-evolution operator. The contour formulation admits much larger time steps than conventional propagators while preserving accurate spectral information (arXiv:2409.07354).
  • Non-Hermitian / transcorrelated Hamiltonians as a first-class case. PyMES treats similarity-transformed Hamiltonians on equal footing throughout — essential for TC methods where conventional Hermitian solvers don’t apply directly.
  • Systems. A 3D uniform electron gas is built in; arbitrary molecules and solids enter via the FCIDUMP interface (read from PySCF or any other electronic-structure package that exports it).
  • Tech stack. Python, NumPy, SciPy, PySCF, h5py.

Engineering

  • Tensor-heavy numerical code. The coupled-cluster contractions that drive every method here are dense numerical linear algebra in Python on top of NumPy/SciPy — broadcasting, memory layout, contraction order, and profiling all matter.
  • A multi-method framework. Integrals, solvers, and analysis are cleanly separated so that new methods (a new excited-state solver, a new Hamiltonian variant) drop in without disturbing existing ones.
  • Open-source maintainership. Several hundred commits across multiple contributors, versioned releases, and dependency management.

Published methods built on PyMES

  • The contour-integral / real-time EOM-CCSD method — Liao 2024, arXiv:2409.07354.
  • TC-CC validation on the uniform electron gas — Liao et al., Phys. Rev. Research (2021).