Chem-DSE-21: Computational Chemistry — DU B. Sc. Life Science (Chemistry) Previous Year Question Papers

DSE: Computational Chemistry B.Sc. (Prog.) with Chemistry Major 3. Attempt FOUR questions in all. a. b. State the Born-Oppenheimer approximation. Give any two points to justify why this approximation is the comerstone of computational chemistry. Construct the z-matrix of c2H2 using appropriate labels. c. What is geometry optimization? Should calculations from first principles (¢b -z.jiz.Jz.a)bealwayspreferredoversemi-empiricalcalculationswhichmakeuseof experimental data? Why/Why not? d. Formulate the Hamiltonian for a hydrogen atom, and explain the terms involved. Which term in the Schr6dinger equation makes it difficult to solve for multi-electron systems? e. What are basis sets? An crb-7.J2z.}z.a calculation on C2H4 involves a total of 14 basis functions, while a semi-empirical calculation on it involves only 12 basis functions. Justify. f. Out of the force-field methods AMBER, CHARMM, MM3, OPLS, choose which one is the most accurate way of modeling: (i) Hydrocarbons (ii) Bulk Liquids (il) Proteins and Nucleic Acids g. Write the Lennard-Jones potential energy function used to represent non-bonded interactions between two atoms in a molecular mechanics force-field. Also draw the relevant plot. 2. a. De-abbreviate the following terms. iv. GTO v. CNDO b. Write the expression for a typical "molecular mechanics" potential energy (5) function. Predict the number of bond-stretching, angle bending, torsional and non-bonded interaction terms for the propane molecule in a typical force-field. Explain the origin of the bond-stretching and angle bending terms. c. What is a Hessian? How can you characterize a stationary point as a minimum, (5) a maximum or a saddle point from the eigenvalues of a Hessian? 3. a. Match the quantum-mechanical methods in Column A to their approach in in your answer book. Column B to make correct complete/meaningful pairs. Write the complete pair (5) Column A Column B i) HF a) Electron-density based rather than wavefunction ii) MP2 b) Linear combination of electron promotion configurations iii) DFT c) Mean-field (average-field) approximation method iv) CI d) Highly accurate correlation method v) CC e) Post Hartree-Fock method including perturbation correction b. c. With the help of a potential energy diagram of#-butane ®otential energyvs C-C-C-C torsional angle), explain the stability of various confomations of #-butane. (5) Mention the underlying principle of DFT. What makes DFT caloulations (5) computationally less expensive than wavefunction-based aG j.#z.fz.a methods? State the strengths and limitations of DFT. 4. a. b. In MD simulation, how does the Leap-Frog algorithm work? List its two (5) Definethe term "parametrization" in semi-Empirical methods. Explain any one semi-empirical method in detail. (5) c. Distinguish between slater-type and Gaussian-type orbitals. (5) 5. a. What is a Potential Energy Surface? How many dimensions are needed to (5)