The Handy Chemistry Answer Book (2014)

IF YOU CAN IMAGINE IT

What is the focus of theoretical chemistry?

Theoretical chemistry is a branch of chemistry that, as the name suggests, develops and applies theories to explain chemical observations and also to make predictions about things chemists cannot directly study by experiment. Theoretical chemists work on a wide variety of problems that cover pretty much all of the other branches of chemistry. Two of the major subfields within theoretical chemistry are electronic structure theory and molecular dynamics.

What is electronic structure theory?

Electronic structure theory is an area of theoretical chemistry that is focused on calculating the arrangement and energies associated with configurations of electrons in molecules. This can include predicting the structure of a molecule, the most probable arrangement of its electrons, its reactivity, and different excited states of the molecule. For reasons we won’t go into, this is not an easy task, and the electronic structure of molecules cannot be solved exactly, even with very powerful computers. Most theoretical chemists working in this area develop approximations for calculating the true electronic structure of molecules and on testing these approximations against available experimental data to continue to improve existing methods. The electronic properties of molecules play a vital role in determining their stability and reactivity, so despite being challenging, it’s a problem well worth trying to solve.

What molecular properties do theoretical chemists try to calculate?

Theoretical chemists try to calculate pretty much every molecular property there is! If there’s a property we’ve discussed somewhere in this book, odds are that a theoretical chemist has worked on ways to calculate its value for atoms or molecules.

How much error is associated with electronic structure theory calculations?

There can be a pretty large amount of error associated with these calculations, and the main goal is to keep the errors as consistent as possible and to obtain properties via differences in calculations. For example, the energy of the metal–carbon bond in Cr(CO)₆ would be calculated by comparing the energy calculated for Cr(CO)₆ to that calculated for Cr(CO)₅ and CO separated from one another at infinite distance.

What is a molecular dynamics simulation?

Molecular dynamics simulations are a computational model of a collection of molecules that interact with one another under a specified set of conditions (temperature, pressure, etc.). While electronic structure theory calculations typically only involve one or a few molecules, molecular dynamics simulations can involve hundreds or thousands of molecules all at once. The purpose of molecular dynamics simulations is typically to investigate how molecules interact and react while surrounded by a collection of other molecules. While the energies of individual molecules can be investigated by electronic structure theory, a molecular dynamics simulation allows theoretical chemists to also study how molecules are influenced by their surroundings. These effects can be especially important in liquids, where solvent molecules can have a significant impact on reactivity.