Chemistry Reference
In-Depth Information
function
335,336
(TDELF), which allows one to compute the time-resolved
formation, modulation, and breaking of chemical bonds, thus providing a
visual understanding of the dynamics of excited electrons, as, for example,
in Ref. 337. The natural way of calculating the TDELF is from the TDKS
orbitals.
High harmonic generation (HHG) is the production of many harmonics
(sometimes hundreds) of the input frequency from medium intensity lasers.
Here TDDFT calculations have been successful for some atoms
338-340
and molecules.
341,342
Several experiments have used the HHG response of
molecules to determine their vibrational modes,
343
and calculations have
been performed using traditional scattering theory.
344
If HHG response
develops into a new spectroscopy, the electron scattering theory discussed
above may be of utility to treat large molecules.
Multiphoton ionization (MPI) occurs when an atom or molecule loses
more than one electron in an intense electromagnetic field. It was discovered
to be a nonsequential process, meaning that the probability of double ioniza-
tion can be much greater than the product of two independent ionization
events, thus leading to a ''knee'' in the double ionization probability as a func-
tion of intensity.
345-347
TDDFT calculations have been unable to reproduce
this knee accurately, and it has been shown that a correlation-induced deriva-
tive discontinuity is needed in the time-dependent KS potential
186
for the
method to give proper results.
Above-threshold ionization (ATI) refers to the probability of ionization
when the laser frequency is below the ionization potential, i.e., it does not
occur in linear response.
348,349
Both ATI and MPI require knowledge of the
correlated wave function, which is not directly available in a KS calculation,
hence neither are given well in a TDDFT calculation.
Because the ionization threshold plays a crucial role in most strong field
phenomena, Koopman's theorem, which relates the energy level of the KS
HOMO to the ionization energy, should be satisfied. As standard functionals
fail to satisfy Koopman's theorem due to their poor potentials (see Figure 3),
this suggests the use of self-interaction free methods such as OEP
159,174
or
LDA-SIC.
The field of quantum control
350
involving the femto-second control of
chemical bonding has concentrated mainly on the motion of the nuclear
wave packet on a given set of precalculated potential energy surfaces. With
the advent of atto-second pulses, control of electronic dynamics has come
within reach. A marriage of optimal control theory together with TDDFT
appears to be the ideal theoretical tool to tackle these problems.
351,352
The
ability of TDDFT to predict the coherent control of quantum wells using
terahertz lasers has been shown,
353-355
although, once again, the lack of access
to the true wave function is a difficulty. Many difficulties and challenges
remain in order to develop a general-purpose theory, including the coupling
between nuclei and electrons.
356-358
