Laser Induced Photochemical Reaction Simulations And The Implement Programs Of Semiempirical MRCI With Analytic Gradients

The dissertation can be divided into two parts.One part includes chapter 1,2 and 3.In this part,we introduce a semiclassical nonadiabatic molecular dynamic simulation--a semiclassical electron-radiation-ion dynamic approach(SERID) with its applications into the research of the laser induced photochemical reactions,which involve the photodissociation of cyclobutane and the nonradiative deactivation of adenine.The other part includes chapter 4 and 5.In this part,we present a new implementation of a semiempirical multireference configuration interaction with single and double excitations (MRCISD) with analytic gradients.First of all,We briefly show the theoretical background of molecular dynamic and point out the merit and demerit of SERID.In addition,we display the flow chart of a SERID program,which is provided by R.E.Allen and Yusheng Dou.Secondly,we report the SERID simulations for the photodissociation of cyclobutane into two molecules of ethylene.The results clearly show that the reaction processes by two steps.The first step is the C -C bond breaking of Cyclobutane and form the intermediate, tetramethylene diradical,then in the second step,the C-C bond in tetramethylene breaks and two ethylenes move away from each other.The reaction path is in a good agreement with experimental observation.In order to explain the results,the CASSCF/MRPT2 methods are employed to calculate the potential energy curves(PECs) of the ground state and some low-lying excited states along the path determined from the simulation.On the other hand,the PECs are constructed as a function of C-C bond distance but optimizing remaining internal coordinates by employing CASSCF/MRCISD method.These results reveal that the tetramethylene intermediate diradical is actually formed,but on the PECs of 1³A state.Thirdly,a realistic dynamics simulation study of the ultrafast radiationless deactivation of 9H-adenine is performed employing the SERID.The simulation finds that the excited molecule decays to the electronic ground state through two different radiationless pathways:one decay channel involves the out-of-plane vibration of the amino group while the other decay strongly associates with the deformation of the pyrimidine at the C₂ atom.In order to explain the results,the CASSCF methods are employed to calculate the potential energy curves(PECs) of the ground state and four low-lying excited states along the path determined from the simulation.These calculations reveal that the excited molecule decays to the electronic ground state from the lowest excited state ¹Ï€Ï€^*(¹L_α) on both two paths.In addition,the determined lifetime of the excited state obtained is in agreement with the experimental observation.On the other hand,we introduce the theoretical background of semiempirical MRCI and analytic gradients.Firstly,we briefly show the semiempirical methods and recent implementations,focus on the NDDO method and its parameter improvement.Secondly, the graphical unitary group approach(GUGA) has been introduced to calculate MRCISD and its approximation,double contracted CI(DCCI),including the hole-particle symmetry and a mixed driven model for computing coupling coefficients.In addition,the theoretical background of analytic gradient is also briefly presented.After that we first report the improved algorithms,and then employ them to implement the programs of semiempirical MRCI and analytic gradients.The reduced CI density matrices are very crucial to the implementation of analytic gradients,because they are difficult to calculate.Therefore we use the new algorithms based on the hole-particle symmetry to reduce the calculating time. Our benchmark calculations indicate that our program is not only efficient but also without bottleneck.