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{{Infobox software
| name                 = QMC@Home
| logo                 = QMC@Home.gif
| logo caption         = QMC@Home screensaver showing a molecular calculation
| description          = QMC@Home was a completed quantum chemistry BOINC volunteer computing project hosted by the University of Münster that used Diffusion Monte Carlo to calculate noncovalent molecular interaction energies.

| status               = Completed
| category             = Chemistry
| compute              = CPU

| developer            = Martin Korth
| author               = Martin Korth, Arne Lüchow, Stefan Grimme
| sponsor              = University of Münster; later Ulm University
| released             = 23 May 2006
| completed            = c. 2014

| operating system     = Windows, Linux, macOS

| website              = {{URL|http://qah.uni-muenster.de/}}
}}

'''QMC@Home''' ('''Quantum Monte Carlo at Home''') was a [[volunteer computing]] project running on the [[Berkeley Open Infrastructure for Network Computing|BOINC]] platform, dedicated to developing and applying [[Quantum Monte Carlo]] (QMC) methods for use in [[quantum chemistry]]. Originally hosted at the [[University of Münster]], Germany, with participation from the [[Cavendish Laboratory]] at the University of Cambridge, the project allowed volunteers worldwide to donate idle computing cycles to help calculate the electronic structure and [[molecular geometry]] of small molecules.<ref name="wikipedia">{{cite web |url=https://en.wikipedia.org/wiki/QMC@Home |title=QMC@Home |website=Wikipedia |access-date=2026-06-12}}</ref> The project began beta testing on 23 May 2006 and ran until approximately 2014, leaving behind a meaningful scientific legacy in computational chemistry.

[[File:QMC@Home.gif|thumb|290px|The QMC@Home screensaver visualizing an adenine–thymine base pair calculation, one of the project's core research targets.]]

== Background and motivation ==

Interactions between molecules govern nearly every process in chemistry and biology, from the folding of proteins to the stacking of [[DNA]] base pairs. In principle, [[quantum chemistry]] can predict these interactions precisely by solving the electronic [[Schrödinger equation]]:

:<math>\hat{H}\Psi = E\Psi</math>

where <math>\hat{H}</math> is the Hamiltonian operator, <math>\Psi</math> is the many-electron wavefunction, and <math>E</math> is the total energy of the system. However, exact solutions are computationally intractable for all but the smallest atoms. For decades, chemists relied on approximate methods such as [[Density functional theory|DFT]] and [[Coupled cluster|CCSD(T)]], the latter often called the "gold standard" of quantum chemistry.

[[Quantum Monte Carlo]] methods offer a complementary route. Rather than expanding the wavefunction in a basis set, QMC uses stochastic sampling to evaluate integrals in a high-dimensional space, enabling in principle a more systematic approach to the exact solution. The key variant used by QMC@Home was [[Diffusion Monte Carlo]] (DMC), in which an ensemble of random walkers propagates through electron configuration space in imaginary time, converging toward the electronic ground state.<ref name="korth2008">{{cite journal |last1=Korth |first1=Martin |last2=Lüchow |first2=Arne |last3=Grimme |first3=Stefan |year=2008 |title=Toward the Exact Solution of the Electronic Schrödinger Equation for Noncovalent Molecular Interactions: Worldwide Distributed Quantum Monte Carlo Calculations |journal=Journal of Physical Chemistry A |volume=112 |issue=10 |pages=2104–2109 |doi=10.1021/jp077592t |pmid=18201073 |bibcode=2008JPCA..112.2104K}}</ref>

DMC scales favorably with system size, roughly as <math>\mathcal{O}(N^3)</math> for <math>N</math> electrons, compared to the <math>\mathcal{O}(N^7)</math> of CCSD(T). This favorable scaling makes QMC an attractive target for parallelization, and crucially for the concept of volunteer computing: each DMC "walker" trajectory is essentially independent, meaning the problem can be split into thousands of separate workunits with minimal communication overhead.<ref name="cen2007">{{cite web |url=https://cen.acs.org/articles/85/i14/Science-People.html |title=Science People |work=Chemical and Engineering News |publisher=American Chemical Society |date=2007 |access-date=2026-06-12}}</ref>

Stefan Grimme, a chemistry professor at the University of Münster, and his graduate student Martin Korth recognized this potential. According to Korth, the idea of a SETI@home-style project for quantum chemistry had circulated half-jokingly in the group for some time before Korth discovered BOINC and set the project in motion.<ref name="cen2007" />

== Project history ==

=== Origins at Münster (2006) ===

QMC@Home entered alpha testing in early 2006 and opened public beta on 23 May 2006, hosted at <code>qah.uni-muenster.de</code>.<ref name="wikidata">{{cite web |url=https://www.wikidata.org/wiki/Q570192 |title=QMC@Home – Wikidata |access-date=2026-06-12}}</ref> The project's founding team comprised Martin Korth, Prof. Arne Lüchow (theoretical chemistry, quantum Monte Carlo expert), and Prof. Stefan Grimme (computational chemistry, specialist in noncovalent interactions).<ref name="korth2008" /> The Cavendish Laboratory at Cambridge contributed expertise in trial wavefunction construction, a key ingredient of DMC calculations.

The project was listed on the official BOINC projects page at Berkeley and rapidly grew an international volunteer community. By February 2010, QMC@Home had roughly 7,500 active participants from 102 countries, collectively contributing approximately 5 [[FLOPS|teraFLOPS]] of computing power.<ref name="boincstats2010">{{cite web |url=https://web.archive.org/web/20100210123416/http://boincstats.com/stats/project_graph.php?pr=qmc |title=Detailed user, host, team and country statistics for QMC@Home |publisher=BOINCstats |date=2010-02-10 |access-date=2026-06-12 |archive-url=https://web.archive.org/web/20100210123416/http://boincstats.com/stats/project_graph.php?pr=qmc |archive-date=2010-02-10}}</ref> A wider estimate of approximately 14,000 volunteers was reported at the time the primary S22 benchmark calculations were completed.<ref name="cen2007" />

=== The S22 benchmark campaign ===

The central scientific campaign of the Münster phase focused on the '''S22 benchmark set''', a standard collection of 22 noncovalently bound molecular dimers compiled by Hobza and colleagues. The set covers three classes of noncovalent interaction:

* '''Hydrogen-bonded''' complexes: ammonia dimer, water dimer, formic acid dimer, formamide dimer, uracil dimer (HB), 2-pyridoxine/2-aminopyridine, and adenine–thymine (Watson–Crick)
* '''Dispersion-dominated''' complexes: methane dimer, ethene dimer, benzene/methane, stacked benzene dimer, stacked pyrazine dimer, stacked uracil dimer, and stacked indole/benzene
* '''Mixed-character''' complexes: ethene/ethyne, benzene/water, benzene/ammonia, benzene/HCN, T-shaped benzene dimer, T-shaped indole/benzene, and the phenol dimer; additionally adenine–thymine (stacked) and phenol dimer

Workunits distributed to volunteers handled individual DMC energy evaluations for these 22 dimers and their constituent monomers. The interaction energy of each complex is defined as:

:<math>E_{\mathrm{int}} = E_{\mathrm{dimer}} - E_{\mathrm{monomer A}} - E_{\mathrm{monomer B}}</math>

Because interaction energies are typically small (a few kcal mol<sup>-1</sup>) relative to the total electronic energies involved (hundreds or thousands of Hartree), achieving sub-kcal mol<sup>-1</sup> accuracy requires enormous statistical precision from the stochastic DMC algorithm, which in turn requires very large numbers of Monte Carlo samples — exactly the kind of computation that volunteer computing can supply.

The resulting 2008 paper in the ''Journal of Physical Chemistry A'' reported worldwide distributed QMC calculations approaching the exact solution of the electronic Schrödinger equation for noncovalent molecular interactions, providing an independent benchmark for the S22 set using DMC.<ref name="korth2008" />

=== A new application: QASINO ===

In March 2010, the project deployed a new application called '''QASINO''', expanding the range of calculations available to volunteers.<ref name="setigermany">{{cite web |url=https://www.seti-germany.de/wiki/QMC@HOME |title=QMC@HOME – SETI.Germany Wiki |date=2013-07-24 |access-date=2026-06-12}}</ref> The project also began exploring [[Density functional theory]] as a parallel computational approach.

=== Relocation and rebrand (2012–2013) ===

By late 2012, Martin Korth had moved his research group from Münster to the [[University of Ulm]] (Institute for Theoretical Chemistry). In September 2012, the project announced it would migrate from <code>qah.uni-muenster.de</code> to a new server, with work planned to restart under a new URL at <code>qmcathome.org</code> around late October 2012.<ref name="boincforum2012">{{cite web |url=https://boinc.berkeley.edu/forum_thread.php?id=7847 |title=QMC News – Project Moving, URL Change |publisher=BOINC message boards |date=2012-09-13 |access-date=2026-06-12}}</ref>

The transition was significantly delayed. The original Münster server went offline in October 2012 and the new site was not operational until July 2013, leaving volunteers without work for several months. When the project resumed under <code>qmcathome.org</code>, it featured four applications:<ref name="boincforum2012return">{{cite web |url=https://boinc.berkeley.edu/forum_thread.php?id=7847 |title=QMC@Home returns (post 49988) |publisher=BOINC message boards |date=2013-07-25 |access-date=2026-06-12}}</ref>

# Three applications related to '''Quantum Medicinal Chemistry''' (abbreviated QMC in the new context)
# One called '''Clean Mobility''' (<code>cleanmobility.now</code>), analyzing new materials for electric vehicle batteries

The rebranded project expanded QMC from "Quantum Monte Carlo" toward also encompassing "Quantum Medicinal Chemistry," reflecting Korth's broadening research interests at Ulm. The Clean Mobility application attracted attention — and some frustration — from volunteers, as its workunits could run for 50 to over 100 hours with infrequent checkpointing.<ref name="boincforum2013wt">{{cite web |url=https://boinc.berkeley.edu/forum_thread.php?id=9455 |title=QMC@home/cleanmobility.now |publisher=BOINC message boards |date=2014 |access-date=2026-06-12}}</ref> Korth acknowledged the issue in correspondence with a volunteer, noting that average runtimes were roughly one day and that improved checkpointing was being worked on.<ref name="boincforum2012" />

=== Final phase and closure ===

The project's forums were never fully restored after the 2013 migration, and statistics exports ceased, making activity tracking difficult. The project gradually wound down around 2014 as Korth's group shifted focus toward publication and new research directions. The <code>qmcathome.org</code> domain was subsequently acquired by an unrelated Italian publication. The BOINC project is now listed among completed projects.

== Scientific methodology ==

=== Diffusion Monte Carlo ===

The primary algorithm used by QMC@Home was fixed-node [[Diffusion Monte Carlo]] (FN-DMC). In this method, the ground-state wavefunction <math>\Psi_0</math> is projected out from a trial wavefunction <math>\Psi_T</math> using imaginary-time propagation:

:<math>\Psi_0 \approx \lim_{\tau \to \infty} e^{-\tau(\hat{H} - E_T)} \Psi_T</math>

where <math>\tau</math> is imaginary time and <math>E_T</math> is an energy offset. The fixed-node approximation enforces the correct fermionic antisymmetry by requiring walkers to respect the nodal surface of <math>\Psi_T</math>, typically obtained from a Hartree–Fock or DFT single Slater determinant. This introduces the '''fixed-node error''', which the research team identified as the primary remaining methodological bottleneck after the S22 calculations were complete.<ref name="cen2007" />

Trial wavefunctions in QMC@Home used the [[Slater-Jastrow]] form:

:<math>\Psi_T = D_\uparrow D_\downarrow \exp(J)</math>

where <math>D_\uparrow</math> and <math>D_\downarrow</math> are Slater determinants for spin-up and spin-down electrons, and <math>J</math> is the Jastrow factor explicitly including electron–electron and electron–nucleus correlation terms.

=== Workunit design ===

Each [[workunit]] represented a set of DMC energy evaluations for a specific molecular geometry. The target runtime was designed to be between 4 and 48 hours on a 2.4 GHz computer, balancing scientific efficiency with volunteer computing practical constraints.<ref name="wikipedia" /> The statistical nature of DMC meant that many independent runs (from many volunteers) had to be averaged to achieve the required precision in the final interaction energies.

== Molecules studied ==

The S22 benchmark system molecules studied by QMC@Home volunteers included dimers of the following species:<ref name="wikipedia" />

{| class="wikitable"
! System !! Dominant interaction type
|-
| Ammonia dimer (1) || Hydrogen bonding
|-
| Water dimer (2) || Hydrogen bonding
|-
| Formic acid dimer (3) || Hydrogen bonding
|-
| Formamide dimer (4) || Hydrogen bonding
|-
| Uracil dimer, HB (5) || Hydrogen bonding
|-
| 2-Pyridoxine / 2-aminopyridine (6) || Hydrogen bonding
|-
| Adenine / thymine WC (7) || Hydrogen bonding
|-
| Methane dimer (8) || Dispersion
|-
| Ethene dimer (9) || Dispersion
|-
| Benzene / methane (10) || Dispersion
|-
| Benzene dimer T-shape (11) || Dispersion
|-
| Pyrazine dimer (12) || Dispersion
|-
| Uracil dimer stacked (13) || Dispersion
|-
| Indole / benzene stacked (14) || Dispersion
|-
| Adenine / thymine stacked (15) || Dispersion
|-
| Ethene / ethyne (16) || Mixed
|-
| Benzene / water (17) || Mixed
|-
| Benzene / ammonia (18) || Mixed
|-
| Benzene / HCN (19) || Mixed
|-
| Benzene dimer parallel (20) || Mixed
|-
| Indole / benzene T-shape (21) || Mixed
|-
| Phenol dimer (22) || Mixed
|}

== Publications ==

The primary peer-reviewed publication arising from QMC@Home's volunteer computing work is:

* {{cite journal |last1=Korth |first1=Martin |last2=Lüchow |first2=Arne |last3=Grimme |first3=Stefan |year=2008 |title=Toward the Exact Solution of the Electronic Schrödinger Equation for Noncovalent Molecular Interactions: Worldwide Distributed Quantum Monte Carlo Calculations |journal=[[Journal of Physical Chemistry A]] |volume=112 |issue=10 |pages=2104–2109 |doi=10.1021/jp077592t |pmid=18201073 |bibcode=2008JPCA..112.2104K}}

This paper has continued to be cited in subsequent benchmark studies of DMC for noncovalent interactions, including recent work on basis set incompleteness errors in fixed-node DMC.<ref>{{cite journal |last1=Nakano |first1=Kousuke |last2=Shi |first2=Benjamin X. |last3=Alfè |first3=Dario |last4=Zen |first4=Andrea |year=2025 |title=Basis Set Incompleteness Errors in Fixed-Node Diffusion Monte Carlo Calculations on Noncovalent Interactions |journal=Journal of Chemical Theory and Computation |doi=10.1021/acs.jctc.4c01631}}</ref>

== Legacy and context ==

QMC@Home was one of several quantum chemistry and molecular simulation BOINC projects of the mid-2000s, alongside [[Rosetta@home]] (protein structure prediction), [[Predictor@home]], [[eOn]] (materials science), and [[AQUA@home]] (quantum adiabatic computing). Its contribution was particularly valuable in demonstrating that stochastic many-body electronic structure calculations — which require enormous numbers of independent samples — are exceptionally well-suited to volunteer computing architectures.

The fixed-node error problem that the team identified at the end of the S22 campaign remains an active area of computational chemistry research as of 2026, with ongoing work on multi-determinant trial wavefunctions and nodal surface optimization. The Korth group's paper continues to serve as a reference benchmark for new DMC approaches applied to noncovalent interactions.

The project was profiled in ''Chemical and Engineering News'' in 2007 as an example of how BOINC-enabled volunteer computing was enabling ambitious quantum chemistry computations that would have been impractical on a single research group's cluster.<ref name="cen2007" />

== See also ==

* [[BOINC]]
* [[SETI@home]]
* [[Rosetta@home]]
* [[Cosmology@Home]]
* [[Quantum Monte Carlo]]
* [[Density functional theory]]
* [[BOINC projects]]

== References ==

{{Reflist}}

== External links ==

* [https://web.archive.org/web/20130901000000*/http://qah.uni-muenster.de/ Original project website (qah.uni-muenster.de) – Internet Archive]
* [https://web.archive.org/web/20131001000000*/http://qmcathome.org/ Relaunched project website (qmcathome.org) – Internet Archive]
* [https://en.wikipedia.org/wiki/QMC@Home QMC@Home – Wikipedia]
* [https://doi.org/10.1021/jp077592t Korth, Lüchow, Grimme (2008) – primary paper (DOI)]

[[Category:BOINC projects]]
[[Category:Completed BOINC projects]]
[[Category:Quantum chemistry]]
[[Category:Volunteer computing]]
[[Category:University of Münster]]