system. Units The units of the rate constant depend on the global order of reaction: If concentration is measured in units of mol·L−1 (sometimes abbreviated as M), then For order (m + n), the rate constant has units of mol1−(m+n)·L(m+n)−1·s−1 (or M1−(m+n)·s−1) For order zero, the rate constant has units of mol·L−1·s−1 (or M·s−1) For order one, the rate constant has units of s−1 For order two, the rate constant has units of L·mol−1·s−1 (or M−1·s−1) And for order three, the rate constant has units of L2·mol−2·s−1 (or M−2·s−1) Plasma and gases Calculation of rate constants of the processes of generation and relaxation of electronically and vibrationally excited particles are of significant importance. It is used, for example, in the computer simulation of processes in plasma chemistry or microelectronics. First-principle based models should be used for such calculation. It can be done with the help of computer simulation software. Rate constant calculations Rate constant can be calculated for elementary reactions by molecular dynamics simulations. One possible approach is to calculate the mean residence time of the molecule in the reactant state. Although this is feasible for small systems with short residence times, this approach is not widely applicable as reactions are often rare events on molecular scale. One simple approach to overcome this problem is Divided Saddle Theory. Such other methods as the Bennett Chandler procedure, and Milestoning are also developed for rate constant calculations. Divided saddle theory The theory is based on the assumption that the reaction can be described by a reaction coordinate, and that we can apply Boltzmann distribution at least in the reactant state. A new, especially reactive segment of the reactant, called the saddle domain, is introduced, and the rate constant is factored: where α is the conversion factor between the reactant state and saddle domain, while kSD is the rate constant from the saddle domain. The first can be simply calculated from the free energy surface, the latter is easily accessible from short molecular dynamics simulations See also Reaction rate Equilibrium constant Molecularity References Chemical kinetics