**Density functional theory (DFT) calculations: The beginner’s guide to unlocking the secrets of the universe (or at least, the secrets of your molecule)**

Are you tired of using trial and error to figure out the properties of your molecule? Want to know why your molecule is so darn stable (or unstable)? Look no further, because Density Functional Theory (DFT) calculations are here to save the day!

First things first, you’ll need to choose a DFT functional. Don’t worry, you don’t have to be a math whiz to do this. A functional is just a fancy way of describing the energy of your system based on the electron density. There are many different functionals to choose from, but some popular options include the local density approximation (LDA), the generalized gradient approximation (GGA), and hybrid functionals. Think of it like choosing between different ice cream flavors, each one has its own unique taste (or in this case, accuracy).

Next, you’ll need to prepare your input file. This is where you’ll enter all the juicy details about your molecule, like its atomic coordinates. Think of it like filling out a dating profile for your molecule, you want to make sure all the important information is included (e.g. number of electrons, spin, etc.).

Now comes the fun part, performing the calculation! You can use software packages like Gaussian, ORCA, and Quantum ESPRESSO to do the heavy lifting for you. Just like baking a cake, you’ll need to wait for the calculations to finish before you can enjoy the final product (which in this case is the energy of your system, the electron density, and the electronic structure).

Once your calculations are done, it’s time to dig into the output. Think of it like opening a present on Christmas morning, you never know what you’re going to get (except this present will give you the vibrational frequencies, electronic excitation energies, and the response of the system to external fields).

Last but not least, it’s time to interpret your results. This is like trying to figure out what your significant other got you for your birthday, it may take some time and effort but it will be worth it in the end. With DFT, you can predict properties like the equilibrium geometry of a molecule, the bond dissociation energy, or the electronic spectrum of a molecule.

In conclusion, DFT calculations may seem daunting at first, but once you understand the basics, it’s like riding a bike. With a little practice and a lot of patience, you’ll be able to unlock the secrets of your molecule in no time!

**Background to DFT**: Density Functional Theory (DFT) is a powerful computational method used to study the electronic properties of atoms, molecules, and materials. It is based on the density of electrons, rather than the wavefunction, making it a more tractable approach for studying large and complex systems. DFT is widely used in chemistry, physics, and materials science for calculating properties such as total energy, electron density, and molecular structure.

**Units:** In atomic units, all energies are in Hartree (1H = 27.2 eV) and all distances in Bohr (1a0 = 0.529 Å). In regular units, 1 H = 27.2eV, 1 eV = 23.06 kcal/mol, 1 kcal = 4.184 kJ/mol = 503K

**Brief History of DFT:**

- 1926: Old DFT was Thomas-Fermi theory and extensions.
- 1950s and 60s: Slater and co-workers develop Xα as a crude KS-LDA.
- 1965: Modern DFT begins with Kohn-Sham equations. By introducing orbitals, get 99% of the kinetic energy right, get accurate n(r), and only need to approximate a small contribution.
- 1965: KS also suggested local density approximation (LDA) and gradient expansion approximation.
- 1993: More modern functionals (GGA’s and hybrids) shown to be usefully accurate for thermochemistry.
- 1998: Kohn and Pople win Nobel prize in chemistry.
- 2010: DFT in materials science, geology, soil science, astrophysics, protein folding, etc.