research papers 676 doi:10.1107/S0108768106016326 Acta Cryst. (2006). B62, 676–688 Acta Crystallographica Section B Structural Science ISSN 0108-7681 Molecular and crystal properties of ethyl 4,6- dimethyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5- carboxylate from experimental and theoretical electron densities V. G. Tsirelson, a * A. I. Stash, b V. A. Potemkin, c A. A. Rykounov, c A. D. Shutalev, d E. A. Zhurova, e V. V. Zhurov, e A. A. Pinkerton, e G. V. Gurskaya f and V. E. Zavodnik b a Mendeleev University of Chemical Technology, 9 Miusskaya Square, 125047 Moscow, Russia, b Karpov Institute of Physical Chemistry, 10 Vorontsovo Pole, 105064 Moscow, Russia, c Chelyabinsk State University, 129 Br. Kashir- inych Street, 454021 Chelyabinsk, Russia, d Lomonosov State Academy of Fine Chemical Technology, 86 Vernadsky Prospect, 119571 Moscow, Russia, e Department of Chemistry, University of Toledo, Toledo, Ohio 43606, USA, and f Engelhardt Institute of Molecular Biology, Russian Academy of Sciences, 32 Vavilova Street, 119991 Moscow, Russia Correspondence e-mail: tsirel@muctr.edu.ru # 2006 International Union of Crystallography Printed in Great Britain – all rights reserved The electron density and electronic energy densities in ethyl 4,6-dimethyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carbox- ylate have been studied from accurate X-ray diffraction measurements at 110 K and theoretical single-molecule and periodic crystal calculations. The Quantum Theory of Atoms in Molecules and Crystals (QTAMC) was applied to analyze the electron-density and electronic energy-density features to estimate their reproducibility in molecules and crystals. It was found that the local electron-density values at the bond critical points derived by different methods are in reasonable agreement, while the Laplacian of the electron density computed from wavefunctions, and electron densities derived from experimental or theoretical structure factors in terms of the Hansen–Coppens multipole model differ significantly. This disagreement results from insufficient flexibility of the multi- pole model to describe the longitudinal electron-density curvature in the case of shared atomic interactions. This deficiency runs through all the existing QTAMC bonding descriptors which contain the Laplacian term. The integrated atomic characteristics, however, suffer noticeably less from the aforementioned shortcoming. We conclude that the electron- density and electronic energy QTAMC characteristics derived from wavefunctions, especially the integrated quantities, are nowadays the most suitable candidates for analysis of the transferability of atoms and atomic groups in similar compounds. Received 7 March 2006 Accepted 3 May 2006 1. Introduction All recently undertaken studies of the electron density in peptides, biomolecules and related compounds eventually face the question ‘how transferable are the characteristics of the electron density in a series of similar compounds as well as among different conformers of the same molecule?’. Some workers suggest using the multipole-model parameters as the transferable moieties (Pichon-Pesme et al. , 1995; Jelsch et al., 2000; Pichon-Pesme et al. , 2004; Koritsanszky et al. , 2002; Volkov, Koritsanszky, Li & Coppens, 2004; Volkov, Li, Korit- sanszky & Coppens, 2004; Lecomte et al. , 2005), while others (Chang & Bader, 1992; Popelier & Bader, 1994; Breneman & Rhem, 1997; Popelier, 1999; O’Brien & Popelier, 1999, 2001; Matta & Bader, 2000, 2002, 2003; Dittrich et al., 2002; Whitehead et al., 2003; Popelier & Aicken, 2003; Dittrich et al., 2003; Cortes-Guzman & Bader, 2004) deal with characteristics arising from Bader’s (1990, 2005) Quantum Theory of Atoms in Molecules and Crystals (QTAMC). The question of whether experimental or theoretical electron densities provide the most reliable basis for such an analysis is also the subject of some discussion (Pichon-Pesme et al. , 2004; Volkov, Korit-