<!--[[Image:FDTD18(2).png]]-->
where ε<submath>∞\varepsilon_{\infty}</submath> is the value of the permittivity at infinite frequency, τ<submath>p\tau_p</submath> is the relaxation time corresponding to the p''th'' pole having the unit of seconds, and ε<submath>\varepsilon_{sp}</submath> is the value of the static permittivity (at DC) corresponding to the p''th'' pole. <math>\Delta \varepsilon_p = \varepsilon_{sp} - \varepsilon_{\infty}</math> represents the change in permittivity due to the p''th'' pole.
Unmagnetized plasmas are typically modeled as Drude materials. The complex permittivity of a Drude material with N poles is given by:
<!--[[Image:FDTD19(1).png]]-->
where ω<submath>p\omega_p</submath> and v<submath>p\nu_p</submath> are the angular plasma frequency and angular collision frequency corresponding to the p''th'' pole, respectively, and both are expressed in rad/s. For an unmagnetized plasma, ε<submath>∞\varepsilon_{\infty} = 1</submath> = 1.
The complex permittivity of a Lorentz material with N poles is given by:
<!--[[Image:FDTD20.png]]-->
where ω<submath>p\omega _p</submath> and delta δ<submath>p\delta_p</submath> are the angular resonant frequency and angular damping frequency corresponding to the p''th'' pole, respectively, and both are expressed in rad/s. Similar to a Debye material, <math>\Delta \varepsilon_p = \varepsilon_{sp} - \varepsilon_{\infty}</math> represents the change in permittivity due to the p''th'' pole.
{| style="width: 600px" border="1" cellspacing="1" cellpadding="1"