Fluid particle is the smallest unit (atoms or molecules) in the fluid phase (gas, liquid or plasma) without breaking any chemical bond. Energy of fluid particle is divided into potential, electronic, translational, vibrational, and rotational energies. The heat (thermal) energy storage in fluid particle is through the temperature-dependent particle motion (translational, vibrational, and rotational energies). The electronic energy is included only if temperature is high enough to ionize or dissociate the fluid particles or to include other electronic transitions. These quantum energy states of the fluid particles are found using their respective quantum Hamiltonian. These are H f, t = −(ħ /2 m)∇, H f,v = −(ħ /2 m)∇ + Γ x /2 and H f, r = −(ħ /2 I f)∇ for translational, vibrational and rotational modes. (Γ: spring constant, I f : the moment of inertia for the molecule). From the Hamiltonian, the quantized fluid particle energy state E f and partition functions Z f [with the Maxwell–Boltzmann (MB) occupancy distribution ] are found as