2MOLCAS as a Development Platform for Quantum Chemistry Software VALERA VERYAZOV, 1 PER-OLOF WIDMARK, 1 LUIS SERRANO-ANDRE ´ S, 2 ROLAND LINDH, 1 BJO ¨ RN O. ROOS 1 1 Department of Theoretical Chemistry, Chemical Center, P.O.B. 124, S-221 00 Lund 2 Departamento de Quı ´mica Fı ´sica, Instituto de Ciencia Molecular, Universitat de Vale`ncia, Dr. Moliner 50, Burjassot, ES-46100 Valencia, Spain Received 5 September 2003; accepted 6 February 2004 Published online 1 July 2004 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/qua.20166 ABSTRACT: This work presents the quantum chemistry package MOLCAS, with emphasis on its usefulness as a platform for developing new quantum chemical codes, and the reader is assumed to be familiar with such a process. The development of new codes for quantum chemistry is a time-consuming job that can be dramatically simplified by using libraries for standard problems (such as calculation of integrals), and tools to surmount computer language and operating system limitations. The MOLCAS quantum chemistry software contains modules for a variety of quantum chemical methods, such as Hartree–Fock (HF), density functional theory (DFT), coupled-cluster (CC), and multiconfigurational (MCSCF) approaches, including second- order perturbation theory. It runs on almost all UNIX-like platforms and contains a single source code for 32- and 64-bit architecture, as well as for serial and parallel execution. The MOLCAS environment allows the user to include, in a simple way, new codes integrated with libraries and other MOLCAS modules. The key features of the MOLCAS environment include a modular structure, an easy-to-use environment, an application programming interface (API) library for solving common computational and system-oriented problems, a solid verification control system, and tools for a distributed development. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem 100: 626 – 635, 2004 Key words: MOLCAS; computational chemistry; software development Introduction C omputational quantum chemistry has come a long way since its development started in the mid-1960s. The first programs were primitive, dif- ficult for the nonexpert to use, and contained few computational methods (mainly Hartree–Fock [1]). They were disseminated mostly through personal contacts with the developers. The development during the following years was, however, quite intense. The programs became more and more com- plex with an increasing number of features, which enabled more accurate studies and extended the areas of application. The possibility for an accurate Correspondence to: V. Veryazov; e-mail: Valera.Veryazov@ teokem.lu.se International Journal of Quantum Chemistry, Vol 100, 626 – 635 (2004) © 2004 Wiley Periodicals, Inc.