Climate Effects of Carbon Taxes, Taking into Account Possible Other Future Climate Measures

Increasing fuel extraction costs and global temperatures make it likely that in the medium-term future, technological or political measures against global warming will be implemented. In assessments of current climate policy, possible medium-term future developments, such as backstop technologies, are largely neglected, but such developments may crucially affect policy impacts. If such measures are implemented, a carbon tax introduced now may mitigate climate change to greater effect than recent reflections along the lines of the Green Paradox would suggest. Notably, the weak and the strong version of the Green Paradox, related to current and longer-term emissions, may not materialise. Moreover, the tax may allow the demanding countries to extract part of the resource rent, further increasing its desirability.Climate change policy, greenhouse gas tax, carbon tax, Green Paradox, anticipation effects, exhaustible resources, fossil fuels market, backstop technology, uncertainty, resource rent.

Approximation schemes for the study of multi-band Gutzwiller wave functions

The minimum of the Gutzwiller energy functional depends on the number of parameters considered in the variational state. For a three-orbital Hubbard model we find that the frequently used diagonal Ansatz is very accurate in high-symmetry situations. For lower symmetry, induced by a crystal-field splitting or the spin-orbit coupling, the discrepancies in energy between the most general and a diagonal Gutzwiller Ansatz can be quite significant. We discuss approximate schemes that may be employed in multi-band cases where a minimization of the general Gutzwiller energy functional is too demanding numerically.Comment: 8 pages, 8 figure

H\"uckel--Hubbard-Ohno modeling of π\boldsymbol{\pi}-bonds in ethene and ethyne with application to trans-polyacetylene

Quantum chemistry calculations provide the potential energy between two carbon atoms in ethane (H$_3$C$-$CH$_3$), ethene (H$_2$C$=$CH$_2$), and ethyne (HC$\equiv$CH) as a function of the atomic distance. Based on the energy function for the $\sigma$-bond in ethane, $V_{\sigma}(r)$, we use the H\"uckel model with Hubbard--Ohno interaction for the $\pi$~electrons to describe the energies $V_{\sigma\pi}(r)$ and $V_{\sigma\pi\pi}(r)$ for the $\sigma\pi$ double bond in ethene and the $\sigma\pi\pi$ triple bond in ethyne, respectively. The fit of the force functions shows that the Peierls coupling can be estimated with some precision whereas the Hubbard-Ohno parameters are insignificant at the distances under consideration. We apply the H\"uckel-Hubbard-Ohno model to describe the bond lengths and the energies of elementary electronic excitations of trans-polyacetylene, (CH)$_n$, and adjust the $\sigma$-bond potential for conjugated polymers.Comment: 10 pages, 7 figures, 3 table