PSI3 reads input from a text file, which can be prepared in any standard text editor. The default input file name is input.dat and the default output file name is output.dat. So that you can give your files meaningful names, these defaults can be changed by specifying the input file name and output file name on the the command line. The syntax is:
psi3 input-name output-name
PSI3 is a modular program, with each module performing specific tasks and computations. Which modules are run for a particular computation depends on the type of computation and the particular keywords specified in the input file. All keywords in PSI3 use the structure keyword = value, where values may be strings, booleans, integers, or real numbers. If the value is a string which contains a special character (such as a space or a dash) you must enclose the string in double quotation marks. You can give keywords in the input file for specific modules; however, in the first few examples, we will place all our keywords in one section of our input file called psi. Generally, every module you run during your computation will read the keywords in psi, so you can place all your keywords in this section if you choose to do so.