eT 2.0: An efficient open-source molecular electronic structure program

TL;DR

eT 2.0 delivers a highly modular, open-source electronic structure package that extends beyond traditional coupled cluster methods to include strong light-matter coupling, multilevel and multiscale approaches, and real-time electron dynamics. The work highlights substantial performance gains over eT 1.0 via redesigned solvers, memory optimizations, and efficient Cholesky-based integral handling, while broadening capabilities to QED methods, CV S spectroscopy, and CC-in-HF/MLCC schemes. Together, these advances enable accurate, scalable simulations of complex systems under cavities, plasmonic environments, and large active spaces, with future plans for TD-DFT, RIXS/XES, and GPU acceleration. The combination of open development, rigorous testing, and cross-method integration positions eT 2.0 as a versatile platform for cutting-edge electronic structure research with practical impact on spectroscopy, photochemistry, and polaritonic chemistry.

Abstract

The eT program is an open-source electronic structure program with emphasis on performance and modularity. As its name suggests, the program features extensive coupled cluster capabilities, performing well compared to other electronic structure programs, and, in some cases, outperforming commercial alternatives. However, eT is more than a coupled cluster program; other models based on wave function theory (such as full and reduced space configuration interaction and a variety of self-consistent field models) and density functional theory are supported. The second major release of the program, eT 2.0, has specialized functionality for strong light-matter coupling conditions. In addition, it includes a wide range of optimizations and algorithmic improvements, as well as new capabilities for exploring potential energy surfaces and for modeling experiments in the UV and X-ray regimes. Molecular gradients are now available at the coupled cluster level, and high-accuracy spectroscopic simulations are available at reduced computational cost within the multilevel coupled cluster and multiscale frameworks. We present the modifications to the program since its first major release, eT 1.0, highlighting some notable new features and demonstrating the performance of the new version relative to the first release and to other established electronic structure programs.

Figures (12)

• Figure 1: Dispersion of the excitation energies of furan using the aug-cc-pVDZ basis in an optical cavity, as a function of the cavity frequency, $\omega$. On the top panel of the image, we display the orbital transition mainly featured in the excitation. On the bottom panel of the figure, we show the formation of the lower and upper polaritons.
• Figure 2: Dispersion of the ground state energy of LiH using the aug-cc-pVDZ basis with respect to the light-matter coupling, $\lambda$. While QED-CCSD clearly outperforms QED-HF and SC-QED-HF, all the implemented methods capture the qualitative behavior of the function.
• Figure 3: Dispersion of the ground state energy of LiH using the aug-cc-pVDZ basis with respect to the cavity frequency, $\omega$. The QED-HF energy is completely independent of $\omega$ while SC-QED-HF, despite being a mean-field approach, reproduces the QED-FCI behavior qualitatively. Excellent agreement is observed between QED-CCSD and QED-FCI results.
• Figure 4: Time evolution of the isodensity surfaces after subtracting the ground state electron density, plotted using UCSF Chimera.Pettersen2004Chimera The calculation was performed with TD-EOM-CCSD/aug-cc-pVDZ. The system is a molecule of p-nitroaniline in the presence of an external electric field. The red surface depicts the isosurface at 0.001 contour level, while the blue one refers to the -0.001 contour level. The interaction with the external electric field induces a migration of electrons that leads to an increase of negative charge on the nitro group while the amino group becomes more positively charged with respect to the ground state.
• Figure 5: An $S_0$ and an $S_1$ minimum in 9H-adenine determined with EOM-CCSD/cc-pVDZ. In the $S_1$ minimum, we observe a puckering of the C2 carbon atom which is not present in the $S_0$ minimum.