where "k" is the rate constant. "A" was referred to as the frequency factor (now called the pre-exponential coefficient), and "E"a is regarded as the activation energy. By the early 20th century many had accepted the Arrhenius equation, but the physical interpretation of "A" and "E"a remained vague. This led many researchers in chemical kinetics to offer different theories of how chemical reactions occurred in an attempt to relate "A" and "E"a to the molecular dynamics directly responsible for chemical reactions.