2.4. Postulate 4 – Time-independent stationary states
A quantum state is a time-independent stationary state if all its observables are independent of time. These states are very important in quantum chemistry. The atomic orbital of an electron and the molecular orbital of an electron in a molecule are time-independent stationary states.
The time-independent Schrödinger equation can be written as follows, that is, static: 
where 
is the energy eigenvalue, and 
is the state vector of the quantum system not as a function of time.
This postulate implies that the wave function must be an eigenfunction for all measurements and corresponding operations that represent the energy. An eigenfunction is a function that remains unchanged when acted upon it by an operator or when a measurement is made.
We use this concept more in Chapter 4, Molecular Hamiltonians.