COMPUTATIONAL CHEMISTRY: A USEFUL (SOMETIMES MANDATORY) TOOL IN MASS SPECTROMETRY STUDIES Manuel Alcamõ Â, Otilia MoÂ, and Manuel YaÂnÄez* Departamento de Quõ Âmica,C-9,Universidad Auto Ânoma de Madrid, Cantoblanco, 28049-Madrid, Spain Received 25 July 2001; revised 17 October 2001; accepted 24 October 2001 I. Introduction . ......................................................................... 196 II. Theoretical Models . .................................................................... 197 A. Hartree-Fock (HF) Theory ............................................................. 197 B. Con®guration Interaction (CI) .......................................................... 197 C. Perturbation Theory . ................................................................. 198 D. Coupled Cluster Theory . .............................................................. 198 E. Quadratic Con®guration Interaction . ...................................................... 199 F. Composite Procedures ................................................................ 199 G. Multicon®guration Interaction Theories .................................................... 199 H. Density Functional Theory . ............................................................ 200 I. CASSCF-DFT Approaches ............................................................. 201 J. Wave Function Analysis . .............................................................. 202 III. Performance of the Model ................................................................ 203 A. Geometries ........................................................................ 203 B. Harmonic Vibrational Frequencies and Infrared Intensities ....................................... 206 C. Energies ......................................................................... 206 D. Entropies ......................................................................... 208 E. Convergency Problems of the Moller Plesset Series . ........................................... 209 F. Radicals and Other Open-Shell Systems . ................................................... 210 G. Dications ......................................................................... 213 IV. Bond Weakening vs. Bond Reinforcement in Ion-Molecule Interactions ................................ 214 A. Dissociative Proton Attachment .......................................................... 219 B. Ionization Effects in Three-Membered Rings ................................................ 221 V. Theoretical Characterization of Elusive or Non-Conventional Structures of Relevance in Gas-Phase Ion Chemist Con®rmed by Experiment ................................................................ 223 A. Oxywater ......................................................................... 224 B. Ammonia Oxide .................................................................... 224 C. Silicon-Containing Molecules ........................................................... 225 D. Fluoroformic Acid . .................................................................. 225 E. Sulfur- and Phosphorus-Containing Molecules ............................................... 225 F. Ylides ........................................................................... 226 G. Ethylenedione . ..................................................................... 227 H. Cumulenes and Heterocumulenes ........................................................ 227 I. Carbenes ......................................................................... 230 J. Hydrogen-Bridged Radical Cations ....................................................... 230 VI. The Role of Non-Classical Structures in Ion-Molecule Interactions ................................... 231 A. Hydrocarbon Protonated Forms . ......................................................... 231 B. Protonated Forms of X 4 (X  P,As, Sb,Bi) and Other Small Clusters . .............................. 233 C. Molecular Planetary Systems ........................................................... 234 Acknowledgments ......................................................................... 235 References .............................................................................. 235 Mass Spectrometry Reviews, 2001, 20 , 195± 245 # 2002 by John Wiley & Sons, Inc. DOI 10.1002/mas.10005 *Correspondence to: Manuel Ya ÂnÄez,Departamento de Quõ Âmica,C-9, Universidad Auto Ânoma de Madrid, Cantoblanco, 28049-Madrid, Spain. E-mail: manuel.yanez@uam.es ÐÐÐÐ Contractgrantsponsor: DGIProject;Contractgrantnumber: BQU 2000-0245.