**HONPAS**
(**H**efei **O**rder-**N P**ackages for **A**b initio **S**imulations)
is an ab initio electronic structure calculation software package for linear scaling first-principles density functional theory (DFT) calculations
of large-scale systems with standard norm-conserving pseudopotentials and numerical atomic orbitals (NAOs) under the periodic boundary conditions.
HONPAS is developed in the framework of the SIESTA methodology and focuses
on the development and implementation of efficient linear scaling algorithms for ab initio electronic structure calculations.

The HONPAS code has a public repository on both github and bitbucket, and you can download it using the web URL: HONPAS_git or git clone it by "git clone git@github.com:xmqin/HONPAS.git".
Please follow the instructions contained in the README.md file for installing and using the code.
If you have any other questions, please email me (xmqin03@ustc.edu.cn).

1. A series of density matrix purification algorithms for the solution of the electronic ground states, including the trace-preserving canonical purification scheme of Palser and Manolopoulos (PM), the trace-correcting purification (TC), and the trace resetting density matrix purification (TRS). The linear-scaling density matrix second-order trace-correcting purification (TC2) algorithm has been extended to perform spin polarized calculations.

2. A NAO2GTO scheme to calculate the electron repulsion integrals (ERIs) and their derivatives. Within this scheme, calculations of both total energy and atomic forces with the hybrid functionals (PBE0, B3LYP and HSE06) are available. More accurate calculations for post-HF methods such as Møller–Plesset second-order perturbation (MP2) theory and coupled cluster theory are under development.

3. A low rank approximation based on the interpolative separable density fitting (ISDF) decomposition to construct a low rank approximation of HFX matrix, which avoids explicit calculations of ERIs and significantly reduces the computational cost.

4. Linear scaling post-SCF calculations for band edge states, doped semiconductors, and the maximally localized Wannier functions (MLWFs).

5. Linear scaling method based on the density matrix perturbation theory (DMPT) to treat electric field in solids. Optical dielectric constant and Born effective charges of insulating solids can be calculated with it. Linear scaling phonon calculations based on DMPT will also be provided.