DIESEL-MP2: A New Program to Perform Large-Scale Multireference-MP2 Computations* PATRICK MUSCH, BERND ENGELS University of Wuerzburg, Institute for Organic Chemistry, Am Hubland, 97074 Wuerzburg, Germany Received 5 October 2005; Accepted 16 January 2006 DOI 10.1002/jcc.20416 Published online in Wiley InterScience (www.interscience.wiley.com). Abstract: This article presents a new MR-MP2 code (Multi-Reference Møller–Plesset 2nd order) suitable for the computation MR-MP2 energies of extended systems with strong near degeneracy effects (e.g., open shell systems). It is based on the DIESEL program package developed by Hanrath and Engels. Due to improved algorithms the new code is able to handle systems with 400 –500 basis functions and more than 100 electrons. The code is made for parallel computers with distributed memory, but can also be run on local machines. It possesses two integral interfaces (MOLCAS, TURBOMOLE). The algorithms are briefly introduced and timings for the Neocarzinostatin chromophore are presented. The efficiencies of the codes obtained with Intel or GNU compilers are compared. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 1055–1062, 2006 Key words: DIESEL-MP2; large-scale computations Introduction Size and complexity of the systems in material or biological science have imposed novel challenging questions to the field of computational chemistry and lead to the successful development of more efficient computational algorithms. Examples are recent de- velopments for MP2, 1,2 coupled cluster, 3 or DFT, 4 which have reduced the computational cost for large-scale systems to a linear- dependency allowing the treatment of systems up to some hundred atoms and up to a few thousand basis functions. Their application contributes to a deeper understanding of the factors that control material properties or enzymatic activities. 5 However, most of the methods mentioned before refer to a single reference, and thus possess only limited accuracy if near degeneracy effects are important. 6 Such effects are present in many systems being involved in material or enzymatic processes, for example, radicals, diradicals, or transition metals with open shells. 7,8 For such problems multireference methods like MR-CI or MR-PT are necessary to obtain reliable results or at least, to validate the single reference methods. The advantage of multireference methods is their great flexi- bility, which enables these approaches to describe reaction paths or excited states very accurately. 9 The main drawback of multirefer- ence methods is their high computational cost compared to single- reference approaches. Several modifications have been proposed to improve the computational efficiency like the individually select- ing MR-CI, 10,11 CASPT2, 12 MR-MPn, 13–16 MCCEPA, 17 or inter- nally contracted MR-CI. 1,18 The ansatz of local correlation treat- ments 2,19 were also developed and implemented for multireference approaches like local MR-CI 2d,20 and local MR-PT2. 21,22 The computational cost of the latter approaches was reduced to a certain degree, but a reliable estimate on the errors coming along with this approximation has not yet been assessed for reaction paths or for systems with a difficult electronic or a bulky geomet- rical structure. 23 For large-scale multireference computations, the computation- ally quite efficient MR-MP2 approach seems to be a good choice. In this work we present a new code for this purpose. It is based on the DIESEL program package developed by Hanrath and Engels. 11 The advantage of the new code is its ability to handle several hundred basis functions and the possibility to run it on parallel computers with distributed memory. Additionally further subdivi- sion of the interacting space leads to more efficient algorithms. This article consists of a brief introduction to the used MR-MP2 approach, followed by a description of the matrix element com- Correspondence to: B. Engels; e-mail: bernd@chemie.uni- wuerzburg.de *Dedicated to Professor Volker Staemmler on the occasion of his 65 th birthday. Contract/grant sponsor: the Deutsche Forschungsgemeinschaft (in the framework of SFB 410) Contract/grant sponsor: the Volkswagen foundation © 2006 Wiley Periodicals, Inc.