1 Introduction

1.1 About This Manual

(May 16, 2021)

This manual is intended as a general-purpose user’s guide for Q-Chem, a modern electronic structure program. The manual contains background information that describes Q-Chem methods and user-selected parameters. It is assumed that the user has some familiarity with the Unix/Linux environment, an ASCII file editor, and a basic understanding of quantum chemistry.

After installing Q-Chem and making necessary adjustments to your user account, it is recommended that particular attention be given to Chapters 3 and 4. The latter, which describes Q-Chem’s self-consistent field capabilities, has been formatted so that advanced users can quickly find the information they require while supplying new users with a moderate level of important background information. This format has been maintained throughout the manual, and every attempt has been made to guide the user forward and backward to other relevant information so that a logical progression through this manual is not necessary.

Documentation for IQmol, a graphical user interface designed for use with Q-Chem, can be found on the www.iqmol.org websitge. IQmol functions as a molecular structure builder, as an interface for local or remote submission of Q-Chem jobs, and as a post-calculation visualization program for densities and molecular orbitals.

1.1.0.1 Chapter Summaries

Ch. 1:

General overview of Q-Chem’s features, contributors, and contact information.

Ch. 2:

Procedures to install, test, and run Q-Chem on your machine.

Ch. 3:

Overview of the Q-Chem command-line input.

Ch. 4:

Running ground-state self-consistent field calculations.

Ch. 5:

Details specific to running density functional theory (DFT) calculations.

Ch. 6:

Running post-Hartree-Fock correlated wave function calculations for ground states.

Ch. 7:

Running calculations for excited states and open-shell species.

Ch. 8:

Using Q-Chem’s built-in basis sets, or specifying a user-defined basis set.

Ch. 9:

Using Q-Chem’s effective core potential capabilities.

Ch. 10:

Options available for exploring potential energy surfaces, such as determining critical points (transition states and local minima on a single surface, or minimum-energy crossing points between surfaces) as well as ab initio molecular dynamics.

Ch. 11:

Molecular properties and a posteriori wave function analysis.

Ch. 12:

Methods for molecules in complex environments, including implicit solvation models, QM/MM models, the Effective Fragment Potential, and density embedding.

Ch. 13:

Fragment-based approaches for efficient calculations on large systems, calculation of non-covalent interactions, and energy decomposition analysis.

App. A:

Overview of the Optimize package used for determining molecular geometry critical points.

App. B:

Overview of the AOInts library, which contains some of the fastest two-electron integral code currently available.

App. C:

Quick-reference section containing an alphabetized list of job control variables.