Conventions

Units

• Coordinates are provided in atomic units (Bohr) unless otherwise noted.

Arrays

• Numerical arrays are NumPy ndarray (double precision, float64) unless otherwise specified.

• The code uses row-major NumPy arrays (default in NumPy).

Implementation-specific details

• J matrix uses scale=0.5, matching the PySCF RHF convention.

• Internally, contracted Gaussian shells are unit normalized. In the Python interface, the additional Racah factors are applied to Cartesian shells by default to match PySCF’s convention.

• The library supports spherical and Cartesian GTO basis sets with segmented contraction. General contracted and mixed SP-type shells are not supported and should be replaced with segmented contraction analogs.

• Gradients are provided per AO; they are not contracted to atoms. It’s the user’s responsibility to assemble atomic gradients using an AO-to-atom correspondence map. Only the Cartesian shells are currently supported in the gradient code.