Computational Methods and Molecular Modelling — DU B.Sc (hons) Chemistry Previous Year Question Papers

(d) Calculate the ionization potential (IP) & electron affinity (EA) of the central bond where MO energies of a molecule with 4 pi electrons is given as METHODS & MOLECULAR 2. Attempt six questions in all. 1. (a) Molecular Mechanics (MM) is unable to describe bond breaking processes. Explain. (b) What is parametrizing the force field in molecular mechanics? (c) Geometry optimization is necessary before performing molecular dynamics or quantumchemical calculations. Explain. (d) Calculate the Hessian matrix at the point (1,0) of the following multivariable function : 2. (a) What is the role of periodic boundary conditions (PBC) in simulations? (b) Explain the factors that determine the choice of time step in Molecular Dynamics and describe the problems associated with inappropriate time step selection. Coordinate (IRC) and how it helps trace the reaction pathway. (b) Explain how Gaussian functions are used to approximate atomic orbitals in minimal basis sets. Give an example. (c) What is meant by the term ‘correlation energy’? What methods can be used to calculate energies including correlation effects? (d) Discuss the importance of antisymmetry principle in quantum chemistry. (3,4,4,4) 8. (a) Vibrational frequencies are calculated to characterize Stationary points in PES, comment. (b) What are electrostatic potential maps? Give their significance. (c) How Zero Differential Overlap (ZDO) approximation in semi-empirical methods reduces the computational cost. 6. (a) Why is the Born-Oppenheimer approximation significant in ab initio methods? (b) One of the functional forms to represent torsional angles in MM force field is the following : What does various symbols represent in this equation? (c) What does a saddle point in a potential energy surface indicate? How will you differentiate between a minimum and a saddle point? (d) What are the advantages of using internal coordinates or z-matrix? Construct a Z-matrix for formaldehyde. (3,4,4,4) (c) What is the concept and purpose of core-electron replacement with (a)s (d) For a system with potential energy E = 3x2 + y2 + 5y, compute the gradient and explain its significance in energy minimization. (3,4,4,4) 3. (a) Write the extended form of the following : (b) Using Hückel MO theory, compute the resonance energies and compare the stability of propenyl anion and radical. The % molecular orbitals are given as : (c) Why is DFT extensively used in computational (b) Distinguish between Relaxed PES and Rigid PES. chemistry? Explain the concept and importance of (c) Why are thermostats and barostat essential for electron density in DFT. simulating real systems? (d) Different computational techniques vary in (d) Semi-empirical methods are faster than ab initio accuracy and computational cost. Given the methods. Explain. (3,4,4,4) methods: MMF, AMI, HF/STO-3G, and B3LYP/ 6-31G*, which method would be most suitable for 5. (a) How do atomic charges influence electrostatic the following applications: