QuantLib convertible bond pricing generates strange delta

Question

I am trying to generate equity delta for convertible bond using QuantLib(version 1.14) functions, but the deltas generated either using a repricing approach or by directly obtaining from the tree(code as below) all generates deltas bigger than 1.

#include <ql/qldefines.hpp>
#ifdef BOOST_MSVC
#  include <ql/auto_link.hpp>
#endif
#include <ql/experimental/convertiblebonds/convertiblebond.hpp>
#include <ql/experimental/convertiblebonds/binomialconvertibleengine.hpp>
#include <ql/time/calendars/target.hpp>
#include <ql/time/daycounters/thirty360.hpp>
#include <ql/utilities/dataformatters.hpp>

#include <boost/timer.hpp>
#include <iostream>
#include <iomanip>

#define LENGTH(a) (sizeof(a)/sizeof(a[0]))

using namespace QuantLib;

#if defined(QL_ENABLE_SESSIONS)
namespace QuantLib {

    Integer sessionId() { return 0; }

}
#endif
template<typename Method>
auto calculate(double underlying)
{
    auto analysis_date = Date(8, May, 2020);
    Real spreadRate = 0.0125;
    Spread dividendYield = 0.0;
    Rate riskFreeRate = 0.03;
    Volatility volatility = 0.3436553822850044;
    Integer settlementDays = 0;
    Integer length = 3;
    Real redemption = 100.0;
    Real conversionRatio = 100 / 12.1; // at the money
    // set up dates/schedules
    Calendar calendar = TARGET();
    Date today = calendar.adjust(analysis_date);
    Settings::instance().evaluationDate() = today;
    Date settlementDate = calendar.advance(today, settlementDays, Days);
    Date exerciseDate = calendar.advance(settlementDate, length, Years);
    Date issueDate = calendar.advance(exerciseDate, -length, Years);
    BusinessDayConvention convention = ModifiedFollowing;
    Frequency frequency = Annual;
    Schedule schedule(issueDate, exerciseDate, Period(frequency), calendar, convention, convention, DateGeneration::Backward, false);
    DividendSchedule dividends;
    CallabilitySchedule callability;
    std::vector<Real> coupons(1, 0.05);
    DayCounter bondDayCount = Thirty360();

    for (Date d = today + 6 * Months; d < exerciseDate; d += 6 * Months)
    {
        dividends.push_back(boost::shared_ptr<Dividend>(new FixedDividend(1, d)));
    }
    DayCounter dayCounter = Actual365Fixed();
    boost::shared_ptr<Exercise> exercise(new EuropeanExercise(exerciseDate));
    boost::shared_ptr<Exercise> amExercise(new AmericanExercise(settlementDate, exerciseDate));
    Handle<Quote> underlyingH(boost::shared_ptr<Quote>(new SimpleQuote(underlying)));
    Handle<YieldTermStructure> flatTermStructure(boost::shared_ptr<YieldTermStructure>(new FlatForward(settlementDate, riskFreeRate, dayCounter)));
    Handle<YieldTermStructure> flatDividendTS(boost::shared_ptr<YieldTermStructure>(new FlatForward(settlementDate, dividendYield, dayCounter)));
    Handle<BlackVolTermStructure> flatVolTS(boost::shared_ptr<BlackVolTermStructure>(new BlackConstantVol(settlementDate, calendar, volatility, dayCounter)));
    boost::shared_ptr<BlackScholesMertonProcess> stochasticProcess(new BlackScholesMertonProcess(underlyingH, flatDividendTS, flatTermStructure, flatVolTS));
    Size timeSteps = 801;
    Handle<Quote> creditSpread(boost::shared_ptr<Quote>(new SimpleQuote(spreadRate)));
    boost::shared_ptr<Quote> rate(new SimpleQuote(riskFreeRate));
    Handle<YieldTermStructure> discountCurve(boost::shared_ptr<YieldTermStructure>(new FlatForward(today, Handle<Quote>(rate), dayCounter)));
    ConvertibleFixedCouponBond americanBond(amExercise, conversionRatio, dividends, callability, creditSpread, issueDate, settlementDays, coupons, bondDayCount,
            schedule, redemption);
    americanBond.setPricingEngine(boost::shared_ptr<PricingEngine>(new BinomialConvertibleEngine<Method>(stochasticProcess, timeSteps)));
    Real npv = americanBond.NPV();
    Real delta = americanBond.delta();
    return std::make_pair(npv, delta);
}

template<typename Method> void calc_sensitivity()
{
    auto spot=10.34;
    auto [npv, delta]= calculate<Method>(spot);
    auto [npv2, delta2]=calculate<Method>(spot*1.01);
    delta2 = (npv2 - npv) / (spot*0.01);
    // write column headings
    Size widths[] =
    { 14, 14, 14, 14 };
    Size totalWidth = widths[0] + widths[1] + widths[2] + widths[3];
    std::string rule(totalWidth, '-'), dblrule(totalWidth, '=');
    std::cout << typeid(Method).name() << std::endl;
    std::cout << dblrule << std::endl;
    std::cout << std::setw(widths[0]) << std::left << "PV0" << std::setw(widths[1]) << std::left << "PV1" << std::setw(widths[2]) << std::left
            << "Tree Delta" << std::setw(widths[3]) << std::left << "Iterative Delta" << std::endl;
    std::cout << rule << std::endl;
    std::cout << std::setw(widths[0]) << std::left << npv << std::fixed << std::setw(widths[1]) << std::left << npv2 << std::setw(widths[2]) << std::left
            << delta << std::setw(widths[2]) << std::left << delta2 << std::endl;

    std::cout << dblrule << std::endl;
}

int main(int, char*[])
{

    try
    {
        boost::timer timer;
        std::cout << std::endl;

        calc_sensitivity<JarrowRudd>();
        calc_sensitivity<CoxRossRubinstein>();
        calc_sensitivity<AdditiveEQPBinomialTree>();

        double seconds = timer.elapsed();
        Integer hours = int(seconds / 3600);
        seconds -= hours * 3600;
        Integer minutes = int(seconds / 60);
        seconds -= minutes * 60;
        std::cout << " \nRun completed in ";
        if (hours > 0)
            std::cout << hours << " h ";
        if (hours > 0 || minutes > 0)
            std::cout << minutes << " m ";
        std::cout << std::fixed << std::setprecision(0) << seconds << " s\n" << std::endl;

        return 0;
    } catch (std::exception &e)
    {
        std::cerr << e.what() << std::endl;
        return 1;
    } catch (...)
    {
        std::cerr << "unknown error" << std::endl;
        return 1;
    }

}

below code for generating the delta in binomialconvertibleengine.hpp:

convertible.initialize(lattice, maturity);
convertible.rollback(time_grid[1]);
auto value_up = convertible.values()[1];
auto value_down = convertible.values()[0];
auto s_up = tree->underlying(1, 1);
auto s_down = tree->underlying(1, 0);
auto delta = (value_up - value_down) / (s_up - s_down);

below is the result:

N8QuantLib10JarrowRuddE
========================================================
PV0           PV1           Tree Delta    Iterative Delta
--------------------------------------------------------
104.455       104.677186    1.985473      2.148005      
========================================================
N8QuantLib17CoxRossRubinsteinE
========================================================
PV0           PV1           Tree Delta    Iterative Delta
--------------------------------------------------------
104.454988    104.673906    1.984574      2.117198      
========================================================
N8QuantLib23AdditiveEQPBinomialTreeE
========================================================
PV0           PV1           Tree Delta    Iterative Delta
--------------------------------------------------------
104.480229    104.704433    1.996642      2.168310      
========================================================

Remark: Should have divided by the conversion ratio, the updated result is as below:
========================================================
PV0           PV1           Tree Delta    Iterative Delta
--------------------------------------------------------
110.041       110.544284    0.530003      0.588715      
========================================================
now the deltas looks nice.

Answer 1

You have a conversion ratio of $100/12.1 \approx 8.26$, so the convertibility is an option on about 8 underlying stocks and the delta scales accordingly. I'm not familiar about the way it's quoted, though. Did you expect it to be the delta for one unit of stock?