Correlated harmonic excitation fields

One method for temporal precnrsor isolation is stored waveform inverse Fourier transform (SWIFT) [40]. In this method, the desired freqnency domain profile (all frequencies except that of the ion of interest) is inversely Fonrier transformed to a time domain waveform. This waveform is then applied to the excite electrodes in the ICR cell and, thns, the precursor ions are isolated in the cell. An alternative techniqne for in-cell isolation is correlated sweep excitation (COSE) [41], also known as correlated harmonic excitation fields (CHEF) [42]. This method involves application of radiofrequency pulses to the excite electrodes. The technique correlates the duration and frequency of the RF-pulses with those appropriate to the ions to be isolated. Both SWIFT and COSE are capable of isolating single isotopomers in peptide and protein ions [43-45]. 

De Koning, L.J. Nibbering, N.M.M. van Orden, S.L. Laukien, F.H. Mass selection of ions in an FT-ICR trap using correlated harmonic excitation fields (CHEF). Int. J. Mass Spectrom. Ion Processes 1997,165, 209-219. 

A theoretical justification of the scaling procedure was given by Pupyshev et al [14]. They compared the force field Fhf obtained in the Hartree-Fock (HF) limit with the force-field Fa obtained in the configuration interaction (Cl) technique, where the electron correlation effects are taken into account by mixing the HF ground state function with electronic excitations from the occupied one-electron HF states to the virtual (unoccupied) HF states. It was assumed that the force constants F01 obtained in the Cl approximation simulate the exact harmonic force field while those, extracted from the HF approximation FHF cast the quantum-mechanical force field F1-"1. It was demonstrated that under conditions listed below ... 

Highly Correlated Single-Reference Studies of the O3 Potential Surface. I. Effects of High Order Excitations on the Equilibrium Structure and Harmonic Force Field of Ozone. 

Semiclassical techniques like the instanton approach [211] can be applied to tunneling splittings. Finally, one can exploit the close correspondence between the classical and the quantum treatment of a harmonic oscillator and treat the nuclear dynamics classically. From the classical trajectories, correlation functions can be extracted and transformed into spectra. The particular charm of this method rests in the option to carry out the dynamics on the fly, using Born Oppenheimer or fictitious Car Parrinello dynamics [212]. Furthermore, multiple minima on the hypersurface can be treated together as they are accessed by thermal excitation. This makes these methods particularly useful for liquid state or other thermally excited system simulations. Nevertheless, molecular dynamics and Monte Carlo simulations can also provide insights into cold gas-phase cluster formation [213], if a reliable force field is available [189]. 

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