Quantum Chemistry Lecture Review

Flashcards covering key concepts from the lecture on BSSE, geometry optimization, potential energy surfaces, bond breaking, frequency calculations, and time-dependent DFT for electronic spectra.

BSSE (Basis Set Superposition Error)

An artificial overestimation of interaction energy when two molecules interact, occurring because an incomplete basis set of one molecule 'borrows' basis functions from the other.

Counterpoise method

A computational technique used to correct for the Basis Set Superposition Error (BSSE) in interaction energy calculations for weak interactions.

Standard Orientation (Gaussian output)

The cleanest way of expressing an optimized structure in Cartesian coordinates within a Gaussian output file, useful for recovering optimized geometry.

E_AA (in Counterpoise method)

The energy of molecule A evaluated using the basis set of molecule A, in the distorted geometry it adopts within a dimer, calculated via a standard single point calculation.

Ghost basis set (Ghost atoms)

Basis functions placed at the atomic positions of an absent atom (represented by 'Bq' in Gaussian), used to simulate the presence of basis sets without their nuclei or electrons to correct for BSSE.

OPT WITH MODREDUNDANT option

A Gaussian option that allows explicit control over specific internal coordinates (bonds, angles, dihedrals) during geometry optimization or scanning.

f flag (Modredundant)

A flag within the Modredundant option that 'freezes' a coordinate, keeping its value constant at the input guess during optimization.

s flag (Modredundant)

A flag within the Modredundant option used to perform a 'scan' of a coordinate, varying its value incrementally over a specified number of steps and amplitude.

Rigid Scan

A type of scan where a specific coordinate is manually changed at each step, and only single point energies are computed without optimizing any other geometrical parameters.

Relaxed Scan

A type of scan where a specific coordinate is varied automatically, and at each step, all other atoms are fully optimized to find the minimal energy configuration under the given constraint.

Potential Energy Surface (PES)

A mathematical representation showing how the energy of a molecular system varies with changes in its atomic coordinates, crucial for studying reactions, bond dissociation, and transition states.

Transition State

A high-energy point (a maximum) on the Potential Energy Surface that represents an unstable configuration through which a chemical reaction progresses, determining the reaction rate.

Homolytic Dissociation

Bond breaking where each atom of the bond takes one electron, leading to the formation of radical species (e.g., H₂ 2H•), typically associated with a triplet state at dissociation.

Heterolytic Dissociation

Bond breaking where one atom takes both electrons from the bond, forming an anion and a cation (e.g., H₂ H⁺ + H⁻), typically associated with a singlet state.

Singlet State

A spin state where all electrons in the system are paired, resulting in a total spin of zero.

Triplet State

A spin state characterized by two unpaired electrons with parallel spins, resulting in a total spin of one.

Frequency Calculation

A Gaussian calculation that computes the second derivatives of the energy with respect to atomic coordinates, yielding vibrational frequencies, IR/Raman spectra, and thermodynamic functions, and verifying if a geometry is a true minimum.

Harmonic Approximation (Frequencies)

A simplification in frequency calculations where the potential energy surface for molecular vibrations is approximated as an ideal parabola, which is fast but neglects anharmonic effects.

Imaginary Frequencies

Frequencies with a negative or imaginary value obtained from a frequency calculation, indicating that the optimized geometry is not a true minimum but rather a maximum or a saddle point on the PES (e.g., a transition state).

Global Minimum

The lowest possible energy point on the entire Potential Energy Surface, representing the most stable equilibrium configuration of a molecule.

FREQ keyword (Gaussian)

A keyword in Gaussian input files used to request a vibrational frequency calculation, often combined with OPT to ensure frequencies are calculated for an optimized structure.

IR Intensity

A measure associated with each vibrational mode, indicating how strongly that mode interacts with infrared radiation, calculated from the change in dipole moment during vibration.

Time-Dependent DFT (TD-DFT)

A computational method based on Density Functional Theory used to study electronic excited states, allowing for the calculation of excited state geometries and electronic spectra (UV-Vis).

TD keyword (Gaussian)

A keyword in Gaussian input files used to perform Time-Dependent DFT calculations, requiring specification of the number of excited states to calculate.

Oscillator Strength

A dimensionless value in TD-DFT calculations that quantifies the intensity of a specific electronic excitation, directly related to the absorbance in an electronic spectrum.

Spin-Forbidden Transition

An electronic transition where the total spin of the system changes (indicated by a non-zero S² value in Gaussian output), making it forbidden by selection rules and thus having very low or zero intensity.

Symmetry-Forbidden Transition

An electronic transition that is forbidden due to the symmetry of the initial and final electronic states (e.g., starting and arriving orbitals having the same symmetry), resulting in zero oscillator strength.