# Introduction¶

## What is the LOSC method?¶

The localized orbital scaling correction (LOSC) is a newly developed method in density functional theory (DFT) to eliminate the delocalization error (DE) of many conventional density functional approximations (DFAs). Bascally, the LOSC method involves a set of localized orbitals (LOs) to construct the local occupation number, the LOSC curvature matrix and the LOSC corrections. The references of LOSC that describes the detailed methodology are listed here.

## What is the LOSC library for?¶

The LOSC library is developed for two goals:

It provides sub-libraries with compatible interfaces to several popular programming languages in quantum chemistry, including C, C++ and Python, for the developers who would be interested to implement LOSC method in their favorite quantum chemistry packages. These sub-libraries provide the functionalities to perform the essential calculations of the LOSC method, such as constructing the LOs, LOSC curvature matrix, and LOSC corrections.

It provides the implementation of LOSC method in an open-source quantum chemistry package, psi4, with the Python interface. If you are a user of pis4, you can directly use this library with psi4 to perform LOSC calculations. See this section.

## References of LOSC¶

- 1
Li, C.; Zheng, X.; Su, N. Q.; Yang, W. Localized Orbital Scaling Correction for System- atic Elimination of Delocalization Error in Density Functional Approximations. Natl. Sci. Rev. 2018, 5, 203−215. 203-215.

- 2
Su, N. Q.; Mahler, A.; Yang, W. Preserving Symmetry and Degeneracy in the Localized Orbital Scaling Correction Approach. J. Phys. Chem. Lett. 2020, 11, 1528−1535.

- 3
Mei, Y.; Chen, Z.; Yang, W. Self-Consistent Calculation of the Localized Orbital Scaling Correction for Correct Electron Densities and Energy-Level Alignments in Density Functional Theory. J. Phys. Chem. Lett. 2020, 11, 23, 10269–10277.