Dual level methods

There is also a large potential in dual level methods that provide a systematic basis for scaling potential function, frequency functions, and other -dependent properties obtained at lower levels of theory with the help of high-accuracy calculations at selected points along the RP. More experience is needed to make scaling a reliable procedure and to obtain in... 

Instead of ignoring the atoms less directly connected to the reacting center, it is also possible to treat them at a lower level of theory than the key central part. This leads to the general class of hybrid methods. Most of these are dual-level techniques, but as mentioned below, it is also possible to use multilevel approaches. In many cases, the lower level method is simply molecular mechanics that together with a QM method for the central region gives QM/MM methods. QM/QM techniques are also possible. In all hybrid methods, as well as choosing the different levels to apply to the different... 

An extension of the dual energy method is the use of triple windows in which three windows are chosen with two overlapping windows of different width having the same upper energy level located below the lower level of the photopeak, which is centered on 511 keV. Data are obtained for the object and a calibration phantom. A calibration factor is calculated from the ratio of counts in the two lower scatter windows for both the calibration phantom and the object. The scatter contribution to the photopeak is then estimated from the calibration factor and the narrower energy window data. This method works well for a wide range of activity distribution and object sizes. 

Multisizer 4). The principle of the symmetric dual-channel design is to have the noise levels for the output signals (Vdi and Vd2 as indicated in Fig. 3) in both gate branches identical, and hence, the noises can be canceled by using a differential amplification mechanism. However, ideal noise subtraction is not possible due to the real-world limitation in fabricating identical dual channels. Furthermore, this dual-channel method will not be able to detect particles when two particles pass the two apertures at the same time because the two signals with the similar amplitude will be subtracted by each other and canceled at the second stage of the differential amplifier. 

Dual-level direct dynamics of the hydroxyl radical reaction with ethane and haloethanes Toward a general reaction parameter method ... 

The IVTST method has been applied successfully to the reactions Cl + CH4 [55], OH + CH4 [18], OH + CD4 [57], and OH + C2H6 [7d]. Because these applications were very successful, and because IVTST can be incorporated into electronic structure codes in a very systematic and general way, IVTST may often be the method of choice for future applications. Hence further development of the method would probably be very useful. Nevertheless we are currently even more enthusiastic about dual-level schemes, which are discussed next. 

Because of cost considerations, researchers have tried to combine low level calculations that are used to follow the RP with high level calculations applied just to the stationary points along the RP. Truhlar and co-workers have worked out appropriate correction functions that adjust the potential, frequencies, moments of inertia, etc. obtained with a low level method to the high level results obtained at the stationary points (and some other points). These dual level descriptions have been found to improve the accuracy of reaction rate calculations."... 

Both the dual-level descriptions and the NDDO-SRP approach have to be applied with care since they can lead to discontinuities and spurious minima in the PES function. Also, there are cases for which the reparametrization of a NDDO method is too difficult to provide more than just a qualitative insight into the reaction dynamics. Clearly, dual-level description and NDDO-SRP methods are only of limited value for a detailed investigation of reaction mechanism, energy transfer, and energy dissipation with the help of the RPH. 

Dual-level dynamics " " refers to dynamics calculations that use two levels of electronic structure theory or two PEFs of different quality. In the VTST/MT context, such methods use a low-level method to calculate the MEP and gather some information along it followed by using a smaller... 

The formula for the interpolation of frequencies in the original meth-od allowed for the possibility of negative frequencies, which was problematic. Therefore, only the updated interpolation method " will be discussed. The 3N — 7 real frequencies calculated using the lower level method are denoted as The dual-level frequencies calculated using the high-... 

Dual-Level Direct Dynamics Method for Reaction Rate Calculations with Inclusion of Multidimensional Tunneling Effects and Validation for the Reaction of H with trans-N2H2. 

Mapped Interpolation Scheme for Single-Point Energy Corrections in Reaction Rate Calculations and a Critical Evaluation of Dual-Level Reaction Path Dynamics Methods. 

The ability of theory to account for the wide range of spin-forbidden reactivity observed in a near-quantitative way means that the same theoretical models can be trusted to give insight into more complex transition metal systems. For these other systems, detailed experimental data are not always present for comparison, and it is not always possible to carry out high-level ab initio computations in order to calibrate DFT methods. Nevertheless, the dual approach of locating MECPs and using NA-TST will clearly be able to provide lots of qualitative and semiquantitative insight into reactivity. 

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