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# 3.3.1 Introduction

(February 4, 2022)

The $molecule section communicates to the program the charge, spin multiplicity, and geometry of the molecule being considered. The molecular coordinates input begins with two integers: the net charge and the spin multiplicity of the molecule. The net charge can be any integer, including 0 for neutral molecules, positive for cations, negative for anions. The multiplicity can be any integer as well (1 for a singlet, 2 for a doublet, 3 for a triplet, etc.). Each subsequent line of the molecular coordinate input corresponds to a single atom in the molecule (or dummy atom), regardless of whether using Z-matrix internal coordinates or Cartesian coordinates. Note: The coordinate system used for declaring an initial molecular geometry by default does not affect that used in a geometry optimization procedure. See Appendix A which discusses the Optimize package in further detail. Q-Chem begins all calculations by rotating and translating the user-defined molecular geometry into a Standard Nuclear Orientation whereby the center of nuclear charge is placed at the origin. This is a standard feature of most quantum chemistry programs. This action can be turned off by using SYM_IGNORE TRUE. Note: SYM_IGNORE = TRUE will also turn off determining and using of the point group symmetry. Note: Q-Chem ignores commas and equal signs, and requires all distances, positions and angles to be entered as Ångstroms and degrees unless the INPUT_BOHR$rem variable is set to TRUE, in which case all lengths are assumed to be in bohr.