master: NEW:

ASH: a multi-scale, multi-theory modelling program

ASH is a Python-based computational chemistry and QM/MM environment for molecular calculations in the gas phase, explicit solution, crystal or protein environment. It's a program for performing single-point calculations, geometry optimizations, nudged elastic band calculations, surface scans, molecular dynamics, numerical frequencies and many other things using a MM, QM, QM/MM or ONIOM Hamiltonian. Interfaces to popular QM codes: ORCA, xTB, PySCF, MRCC, ccpy, Psi4, Dalton, CFour, TeraChem, QUICK. Interface to the OpenMM library for MM and MD algorithms. Interfaces to specialized high-level QM codes like Block, Dice and ipie for DMRG, SHCI and AFQMC calculations. Interfaces to machine-learning libraries like PyTorch, MACE and MLatom for using and training machine learning potentials. Excellent environment for writing simple or complex computational chemistry workflows.

Citation If ASH is useful in your research please cite us: ASH: a Multi-scale, Multi-theory Modeling program <https://onlinelibrary.wiley.com/doi/10.1002/jcc.70359>_ R. Bjornsson*, J. Comput. Chem 2026, 47, e70359.

In case of problems: Please open an issue on Github and we will try to fix any problems as soon as possible.

Installation: See https://ash.readthedocs.io/en/latest/setup.html for detailed installation instructions. A proper ASH installation should usually be done in a conda/mamba environment together with the OpenMM library.

Basic installation via pip:

#Install ASH using pip (default main branch)
pip install git+https://github.com/RagnarB83/ash.git

#Install the NEW (development) branch of ASH
pip install git+https://github.com/RagnarB83/ash.git@NEW

Documentation:

https://ash.readthedocs.io

Development:

ASH welcomes any contributions.

Ongoing priorities:

• Improve packaging
• Prepare for 1.0 release
• Fix more Python faux pas
• Write unit tests
• Improve documentation of code, write docstrings.

Basic example:

from ash import *

coords="""
H 0.0 0.0 0.0
F 0.0 0.0 1.0
"""
#Create fragment from multi-line string
HF_frag=Fragment(coordsstring=coords, charge=0, mult=1)
#Alternative: Create fragment from XYZ-file
#HF_frag2=Fragment(xyzfile="hf.xyz", charge=0, mult=1)

#Create ORCATheory object
input="! r2SCAN def2-SVP def2/J tightscf"
blocks="%scf maxiter 200 end"
ORCAcalc = ORCATheory(orcasimpleinput=input, orcablocks=blocks)

#Singlepoint calculation
Singlepoint(theory=ORCAcalc,fragment=HF_frag)

#Call optimizer
Optimizer(theory=ORCAcalc,fragment=HF_frag)

#Numerical frequencies
NumFreq(theory=ORCAcalc,fragment=HF_frag)

#DFT Molecular dynamics simulation for 2 ps with a 0.001 ps (1 fs) timestep
MolecularDynamics(fragment=HF_frag, theory=ORCAcalc, timestep=0.001, simulation_time=2)

QM/MM example:

from ash import *

# Defining a fragment
fragment = Fragment(pdbfile="system.pdb")
# QM-method and QM-region
qm_orca = ORCATheory(orcasimpleinput="! r2SCAN-3c tightscf", numcores=8)
# MM Theory
omm  = OpenMMTheory(xmlfiles=["charmm36.xml", "charmm36/water.xml", "specialresidue.xml"], 
                    pdbfile="system.pdb", periodic=True)

# QM/MM Theory
qmatoms = [93,94,95,96,97,133,134,135, 2001,2002]
qm_mm = QMMMTheory(qm_theory= qm_orca, mm_theory= omm, fragment=fragment, 
                    qm_charge=-1, qm_mult=6,  qmatoms= qmatoms, printlevel=1)

# Geometry optimization
Optimizer(theory=qm_mm,fragment=fragment, actatoms=qmatoms)
# or Molecular dynamics
MolecularDynamics(fragment=fragment, theory=qm_mm, timestep=0.001, simulation_time=2)