Eigenvalue spacing

Fig. 4.4. Wigner distribution Pw x) (dotted line) and the limiting distribution Pi x) for the normalized eigenvalue spacings of M x M matrices with M —> oo (full line). The function Pl x) was plotted using the data listed in Table 4.3 of Haake (1991).

Figure I. Calculated eigenvalue spacings for d"" generation weakly coupled (a) and strongly coupled (b) Cantor-layered chains, compared to a weakly coupled Fibonacci chain (c).

The harmonic potential is flatter than the anharmonic potential and the eigenvalue spacing is smaller. The harmonic approximation, therefore, overestimates the vibrational partition function compared to the quantum anharmonic partition function (at 400K harmonic = 40.2 anharmonic = 7.8). 

To extract infomiation from the wavefimction about properties other than the probability density, additional postulates are needed. All of these rely upon the mathematical concepts of operators, eigenvalues and eigenfiinctions. An extensive discussion of these important elements of the fomialism of quantum mechanics is precluded by space limitations. For fiirther details, the reader is referred to the reading list supplied at the end of this chapter. In quantum mechanics, the classical notions of position, momentum, energy etc are replaced by mathematical operators that act upon the wavefunction to provide infomiation about the system. The third postulate relates to certain properties of these operators ... 

A key issue in describing condensed matter systems is to account properly for the number of states. Unlike a molecular system, the eigenvalues of condensed matter systems are closely spaced and essentially mfmite in... 

The electronic Hamiltonian and the comesponding eigenfunctions and eigenvalues are independent of the orientation of the nuclear body-fixed frame with respect to the space-fixed one, and hence depend only on m. The index i in Eq. (9) can span both discrete and continuous values. The q ) form... 

Since the electronic eigenvalues (the adiabatic PESs) are uniquely defined at each point in configuration space we have m(0) = m(P), and therefore Eq. (32) implies the following commutation relation ... 

Fig. 9. Two-dimensional sketch of the 3N-dimensional configuration space of a protein. Shown are two Cartesian coordinates, xi and X2, as well as two conformational coordinates (ci and C2), which have been derived by principle component analysis of an ensemble ( cloud of dots) generated by a conventional MD simulation, which approximates the configurational space density p in this region of configurational space. The width of the two Gaussians describe the size of the fluctuations along the configurational coordinates and are given by the eigenvalues Ai.

From a mathematical point of view, conformations are special subsets of phase space a) invariant sets of MD systems, which correspond to infinite durations of stay (or relaxation times) and contain all subsets associated with different conformations, b) almost invariant sets, which correspond to finite relaxation times and consist of conformational subsets. In order to characterize the dynamics of a system, these subsets are the interesting objects. As already mentioned above, invariant measures are fixed points of the Frobenius-Perron operator or, equivalently, eigenmodes of the Frobenius-Perron operator associated with eigenvalue exactly 1. In view of this property, almost invariant sets will be understood to be connected with eigenmodes associated with (real) eigenvalues close (but not equal) to 1 - an idea recently developed in [6]. 

We restrict ourselves to finite-dimensional Hilbert spaces, making H a Her-mitian matrix. We denote the eigenvalues of H q) by Efc(g) and consider the spectral decomposition... 

In the spirit of Koopmans theorem, the local ionization potential, IPi, at a point in space near a molecule is defined [46] as in Eq. (54), where HOMO is the highest occupied MO, p( is the electron density due to MO i at the point being considered, and ej is the eigenvalue of MO i. 

The principal topics in linear algebra involve systems of linear equations, matrices, vec tor spaces, hnear transformations, eigenvalues and eigenvectors, and least-squares problems. The calculations are routinely done on a computer. 

If the hamiltonian is truly stationary, then the wx are the space-parts of the state function but if H is a function of t, the wx are not strictly state functions at all. Still, Eq. (7-65) defines a complete orthonormal set, each wx being time-dependent, and the quasi-eigenvalues Et will also be functions of t. It is clear on physical grounds, however, that to, will be an approximation to the true states if H varies sufficiently slowly. Hence the name, adiabatic representation. 

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