Intro Flashcards
Theory
A rule/set of rules that are postulated to govern the behaviour of physical systems
Law
A rule from a theory that has been proven strong over time
Model
An approximate but well defined procedure of general applicability
The generality of a model can only be established by comparison to experiment for a wide variety of systems
Computation
Use of computers to solve/apply the mathematical equations of a model, for predictive purposes
Molecular Modelling
Essentially synonymous with computational chemistry
(Use of computers to solve/apply the mathematical equations of a model, for predictive purposes)
What must all successful models obey
All successful models must be justifiable by via quantum mechanics as quantum mechanics underlies all of chemistry
However, some models are better understood without resorting to quantum mechanics
What are the computable properties
1) Molecular structures (defined by bond length, bond angle, and torsional angle)
2) Energies and Potential Energy Surfaces (Born-Oppenheimer approximation - separation of nuclear and electron motion)
3) Can locate geometries of important positions on the potential energy surface
4) Other chemical properties (spectral properties like NMR chemical shifts, coupling constants, rovibronic transitions) (thermochemical properties like U, H, S, G of a mol of molecules given T) (unmeasurable properties like partial charge, bond order, aromaticity)
Potential Energy Surface
A hypersurface defined by the potential energy of a collection of atoms over all possible atomic arrangements.
PES has 3(N) -6 or 3(N) - 5 (if linear) dimensions, where N = number of atoms (nuclei)
Important positions on the Potential Energy Surface
Locations {R} where U is minimal: optimal molecular structures = structures of molecular intermediates, reactants, products
Locations {R} of saddle points (negative curve in 1D, positive in rest): transition states (lowest E barriers) for reaction steps
What do computed energies of {R} give rise to?
Ea, delta(E rxn), delta(G), Keq
Discuss cost efficiency of computational methods
Chemical experiments become more expensive over time (chemical waste disposable), but computational methods become less expensive (faster computers, same price)
Computational speed depends on the computer, the theory model, and the code (software)
Computational accuracy should depend only on the model
What does a complete PES provide?
Complete info on all possible structures and all isomerization pathways connecting them
(shows how PE changes with molecular geometry)
