Project 1: Molecular Properties and Comparison of Theoretical Methods

Select any small molecule of your choice, as long as it satisfies the following criteria:

• At least two atoms, and not H₂
• No more than 6 non-hydrogen atoms, plus any number of hydrogens
• Work with the ground electronic state, which must be closed-shell (i.e., no radicals, except by special permission)

Use three different correlation methods:

• Hartree-Fock
• Density Functional Theory (DFT): use whichever flavor of DFT you prefer. Examples: S-VWN (Slater exchange plus VWN correlation, often called LSDA), BLYP (Becke exchange plus LYP correlation), B3LYP, B3PW91, EDF1.
• MP2

For each of these three correlation methods, HF, DFT, MP2, use any two basis sets of your choice. This will give you a total of six different levels of theory. For each level of theory, optimize the molecule to obtain the equilibrium geometry, and at that geometry, obtain the following equilibrium molecular properties:

• Geometry (in angstroms and degrees)
• Energy (in hartrees)
• Harmonic vibrational frequencies (in cm^-1) and infrared intensities (in km mol^-1)
• Dipole moment (in Debye)
• H^o(298 K) (in kcal mol^-1)
• S^o(298 K) (in cal mol^-1 K^-1)
• G^o(298 K) (in kcal mol^-1)

Results: Prepare a table listing your molecular properties at each level of theory and any available experimental results.

Questions for Discussion (VERY IMPORTANT):

1.

For this question, completely ignore any experimental results you may have obtained. This will simulate your doing computations on systems where the experimental data isn't available (which is often the situation when you're doing computations!) Compare the theoretical results to each other. Do they appear to be converging with improved basis set and treatment of correlation, or is there a lot of scatter in the data? Would additional computations be required to obtain reliable predictions? If so, what computations would you recommend? Based on your analysis (and NOT by comparing to experiment!), give a set of ``best'' theoretical predictions and estimate the uncertainties.

2.

Compare the theoretical data, including your ``best estimates'' with the experimental data. Note any significant disagreements and explain them if possible.