# Calculating Bardeen-Cooper-Schrieffer and Magnetic Superstructure Electronic States with $\Theta\Phi$

## TL;DR

The paper introduces the $ heta ext{ extPhi}$ package that enables first-principles calculations of superconducting and magnetic states with complex orderings in multi-band systems, bridging mean-field theory and quantum solid state physics.

## Contribution

It presents a novel computational tool that incorporates BCS superconductivity and magnetic superstructures into first-principles calculations for multi-band systems.

## Key findings

- Demonstrates the implementation with example calculations.
- Allows for first-principles estimates of model Hamiltonian matrix elements.
- Facilitates study of interplay between superconductivity and magnetism.

## Abstract

We propose the $\Theta\Phi$ (Theta-Phi) package which addresses two of the most important extensions of the essentially single-particle mean-field paradigm of the computational solid state physics: the admission of the Bardeen-Cooper-Schrieffer electronic ground state and allowance of the magnetically ordered states with an arbitrary superstructure (pitch) wave vector. Both features are implemented in the context of multi-band systems which paves the way to an interplay with the solid state quantum physics packages eventually providing access to the first-principles estimates of the relevant matrix elements of the model Hamiltonians derived from the standard DFT calculations. Several examples showing the workability of the proposed code are given.